The Pool of London is the highest point on the Thames you can get a proper ocean going ship up to and turn it around.
Thus it is where cargo will move from ships to barges, and all the activity will make it the natural place to cross going north to south.
It is thus the focal point and transport hub for the Thames Valley and the South East Coast, and will be that regions biggest city.
Since the Thames Valley and the South East Coast are by far the most agriculturally productive region in the British Isles pre-potato, and the closest to the continent and the continental trade, the biggest city in that region will be the most populous, wealthiest, and dominant city on Great Britain.
You don't get >10% of England's entire medieval population living in a city just because its the capitol.
Since its the wealthiest and well positioned for continental diplomacy, and an England only polity will probably run communications by sea anyway, its very very likely that a King of England will eventually move their capital there.
A conquering and external polity like the Romans will have their headquarters wherever politics and military necessity dictates, but if you're still having an England controlled by a local ruler-ship, the capital will move to London, and the long term differences will be minimal (beyond butterflies

).