1) London was only made the capital of England by the Normans - before that it was Winchester.
Winchester was capital because it was at the dead centre of a low-lying area ruled by an itinerant court. As soon as it became apparent that Winchester was next to useless as an economic node, focus shifted to London. This is apparent even during King Canute's invasion, when he went straight to London and left Winchester for dessert. The Norman invasion was merely the final nail in Winchester's coffin. The main reasons for its strategic importance during the Anarchy were that, I believe, the Treasury was still located there, and that Stephen's brother and main supporter was the Bishop there, and therefore had networks of patronage.
2) Berlin was not the centre of German Economic activity, nor was Moscow the Centre of Russias (Novgorod/Kiev), Cairo in Egypt (Alexandria), Toledo (Cadiz), etc. They became the centre as much due to the strategic location for control, and the development of economic activity towards them.
The capital will be where it is considered best placed for control of the realm. If the realm is dependent on trade with Europe, it may well be in the south, and isn't guaranteed to be London,
Well, those exceptions are fair enough, I suppose, but they remain exceptions. In a vacuum, the capital is usually synonymous with the major economic centre: see sub-Saharan Africa, for instance. In Germany, Russia and Cadiz, the capitals were chosen because one region achieved dominance over others, and the logical capital of the original state became the capital of the 'imperial' state, so to speak. I don't have to remind you that St Petersburg was capital of Russia for quite a while, as its only warm-water port. In this case, the city with the greatest potential for trade with Europe will naturally become the capital, all other things being equal. That city is unequivocally London.
Potentially, a Northumbrian would rule from Durham/York/Bewick/New City build on the mouth of the Humber/Liverpool/Manchester.
Yes. They would rule Northumbria from there. I think it would be unlikely that this polity would survive many centuries, let alone "centralise" the whole of England, but if they did, they would either do it from London (Hull as a distant second) or England would be unable to reap its full potential. That, however, is an interesting idea that could be fleshed out in this thread: a less prosperous England because Cynehelm the Conqueror or whoever wanted to live in flaming Durham.
but saying the capital will be somewhere without knowing any details of any history is infuriating - especially when talking about the only realm besides Wessex to become Overlord/Bretwalda.
But I understand your concerns - I was trying to describe what I think would be the most-positive-without-being-a-wank vision, which would include changing how the Vikings related to Northumbria.
First of all, thank you for being so courteous; I hope I've come across in a similar manner and I've no interest in starting a debate that isn't entertaining to be involved in.
However, I do have a bit of knowledge of history - not as much as most people on this board, but enough to know that the term 'Bretwalda' was basically meaningless in real terms and was made up after the fact by over-hierarchical churchmen - principally Bede, who would be expected to give the designation to Northumbrian rulers - and popularised by whiggish historians, with Frank Stenton being the last to really take it seriously. Also, gaining the designation wasn't a measure of the kingdom's innate strength, only a crude comparison of the reigning kings of the time.
Also, AElle of Sussex, AEthelberht of Kent and Raedwald of East Anglia were designated Bretwaldas, but that's just a nitpick.