I was specifically referring to the three kingdoms that would exist in Great Britain, not a "balkanization" of the Kingdom of England. The Kingdom of England was relatively new, with Northumbria and Wessex having joined the Kingdom within the last century. I can see they causing trouble for the monarch, but not just breaking apart.
Fair enough, but in fairness deeming it "like the Holy Roman Empire" implies a total disintegration, including England breaking down. And England as a concept had existed for around 150 years if you take Alfred the Great as the first King of England. I agree it hadn't been entirely territorially whole for ever, but England actually unitised surprisingly quickly, and I'm not really aware of any lingering Wessex separatism, and little Northumbrian. In an era of ever-changing boundaries, the English kingdoms seemed to integrate and support the new all-English state with little difficulty. But anyway, with this reunderstanding of your point, then fair enough.
Three kingdoms with independent foreign policies would probably be able to keep each other some kind of balance of power equilibrium through foreign alliances, kind of like Northern Italy in the Middle ages.
This I still find dubious. I just think England is too dominant a state for the British Isles to all keep each other in check. A perpetual alliance of the Welsh, Irish and Scots to all mutually defend each other to prevent England from making land gains is pretty implausible and England is going to be on the offensive sooner or later. It's not outside the boundaries of likelihood that England could take on all three and win, either, and as the centuries pass England is going to reach a state where it is willing and able to join in foreign affairs outside the British Isles - whether that be in France, central Europe or the Baltic/Scandinavia. And if England isn't getting involved on the continent, then unless France is trying to conquer England (for goodness knows what reason) I can't see any reason a foreign power would sign an alliance to keep England from expanding. Simply put, it's delaying the inevitable barring an unexpected catastrophe, and the cost totally outweighs the potential gains for any potential foreign ally. I just don't see what an outside power stands to gain from powerplaying in Britain unless they intend to fight England directly - but if they intend that, why bother allying with a British state, save possibly for Scotland?
For the record, I consider mediaeval northern Italy a poor comparison. For a start northern Italy didn't have one totally dominant state to keep in check. Neither did many of its states have any designs on expansion. Northern Italy was, however, a very rich prize for outside powers to pluck and one that could only keep those outside powers out by banding together and seeking help from more outside powers - however, northern Italy was a significant enough prize that any designs a foreign power had on it drew the ire of other foreign powers - in other words, if any foreign power (France, Spain, Austria) meddled with northern Italian affairs they inevitably drew their rivals in to counterbalance their influence. There's no way the British Isles will draw this level of interest from foreign powers, and there's no way foreign powers would be so very insistent on beating down any state which tries to dominate it (because they're already too late).
OK, so that all was badly worded, but there's a point in there somewhere.