If I was asked in a general sense if Too Fat Lardies Dux Britanniarum rules were usable outside the Dark Ages, I would be inclined to say no. The type of warfare they portray takes place in what we may describe as a 'Balkanised' region, i.e. a notional area which has fragmented into much smaller communities which are either non-cooperative or mutually hostile to each other. In terms of Britain this increasingly stopped being the case as kingdoms coalesced into larger states and eventually into a single kingdom. By the end of the Norman Conquest running a Dux Britanniarum Campaign across the length of Britain would be difficult to present as a true reflection of the times.
That being said, if a much closer look at Medieval society is taken past the 'national' level, right down to the individual families of land-holders and their relations with each other, then a different picture emerges. During the Conquest England was parcelled out to Duke William's followers, who combined with the survivors of the existing English system, became the aristocracy and gentry. While initially these were held at the King's pleasure, eventually they became hereditary holdings of families.
The rules of passing land and property within families, while simple at their core (eldest son inherits), were rendered more complex by landholders making provision for younger children to be granted part of the estates in wills. As females could also inherit if there were no male children, or could have land as part of their dowries (held by her husband in his lifetime), increasingly complicated the matter. Disputes between families (and indeed within them) over inheritances were commonplace well before the 15th Century and in a society where the bulk of income was derived from land, bitter disputes they could be.
The courts were where most of these matters were decided, although it was a long drawn out and often expensive process. The old adage that 'possession is nine tenths of the law' was apparently true, as whoever held the land during the process gained the income from it during that time, which frequently led to physical force being applied to eject the current occupiers. This might range from a simple home invasion by a small group of people and the occupiers being physically thrown out of the property, to a full scale siege, such as what the Pastons faced while attempting to hold on to Caister Castle.
Naturally disputes of this kind resulted in enmity between families, which developed into formal feuds over time. Such ill-feeling was often of long duration and often lasted way beyond anyone's memory of what the original cause for them was forgotten. As most of these families tended to live and interact within quite static communities and regions, a pattern of familial alliances and partisanship also developed, which alone could divide a region into two or more rival and competing parties, with other familial groups drifting between the two camps over time.
The holding of various royal offices, the granting of certain 'liberties' (the right to levy customs or taxes etc.), or election as a member of parliament, were other grounds for enmity, even where land was not involved. The Percy and Neville feud was caused by the Neville's being granted offices relating to the Scottish Border region, a traditionally Percy monopoly at the time, which they saw as an erosion of their power. Within almost every area of England (and indeed Wales where the situation was exacerbated due to the virtual traditional independence of the Marcher Lords), such disputes formed the basis of political life.
Under a strong monarchy such disputes were kept in check and while they were not ended, were kept simmering rather than boiling over. The application of royal power could also be used to check the rise of a family that was growing too influential and of course bring down an 'over-mighty subject' completely, stripping him of all titles and lands and re-distributing them to others. A weak monarchy, such as that of Henry VI, or the political upheaval which resulted from that weakness (The Wars of the Roses), allowed these disputes to erupt into violence.
As competing factions vied for power on the national stage, the support of individual families was essential and if one chose to side with a faction, their opponents were essentially forced to join the other. The Earl of Devon had sided with the Duke of York in 1452, but when his rival Lord Bonville, joined the Duke of York's faction in 1455, he was forced to support the King. The Earl of Salisbury also joined York, giving the Percys no choice but to support the King. The polarisation of regions during this period is almost always as a result of one family choosing to involve themselves in the bigger picture.
The national and local political scenes were intertwined in this period and for each great magnate there were numerous localised families supporting them. It was not a remnant of the old feudal system as such, but a fairly recent phenomenon based on mutual voluntary support. Quite simply a noble derived support from not only his own land and tenants, but also from those knights and gentry which supported him. Those knights and gentry benefited in return by having his influence applied in relation to their own affairs.
Attaching yourself to the right noble might mean that you received financially attractive offices, or that a inheritance dispute was settled in your favour. Most high-ranking nobles had more land than they could manage, so a younger son might be granted the stewardship of a manor, effectively providing him with an income of his own. The down side was of course that if you picked the wrong noble, it could all come crashing down and the family ruined in the process, with occasionally the head of the family facing execution for treason.
The net result of all this was that at a local level the families of an area mostly fell into a political pyramid, or competing pyramids, based on the support of one or more locally influential nobles. These structures were termed 'affinities'. In some few areas a single affinity existed, but usually there were two, or sometimes more. Where one existed there would be a group of families tied to a particular magnate, with the others 'left out in the cold' as it were, with no real political power and just biding their time for things to change.
This is where Dux Britanniarum can be used as a satisfying representation of localised conflict during the Wars of the Roses. The assumption at the start of a campaign is that the most recent local magnate's affinity has been broken up by his death or attainder for treason. Players will represent the heads of local families vying to create their own affinity by gaining the support of other neutral families in the area, while preventing their opponents from doing the same. At the same time they will be attempting to weaken their opponent's support base and income by attempting to seize and hold disputed land, increasing their own power in the process.
The winning player in each campaign 'year' is assumed to have built an affinity to support an un-named local magnate and has become his effective leading supporter in the region. The following campaign year (which is not necessarily a consecutive real year) begins with the political fall of that un-named noble and while the winning player of the previous year has some advantages at the start of the new one, the overall scene resets to its default at the start of the campaign, his opponents being presumed to have been able to have consolidated their losses and to have regained some ground in the intervening period.
Like Dux Britanniarum proper it is an abstract system (the Saxon 'invasion' took place over centuries) and in the real world such disputes only sporadically erupted into the use of armed force on the scale we are talking.