Wasn't winchester the capital of england for a while, or do I just trust ck2 modders too much?
It seems like it would be in a semi-okay position. Nothing as good as london, with the thames and all. Only problem that I can see is that it looks quite easy to attack, where raiding london has only really been done once by a foreign power (aside from france, I suppose, but they're the french.)
Although it wouldn't be as good as london because it's not in as much of a good strategic position for trade, how about york? It's fairly central in britain, has access to the sea via the ouse and foss, which lead to the humber, but it's also inland and easily defendable against amphibious assault via a well-placed fortress (castle?) or two.