Ah. I misread your OP then -- I was under the impression you controlled only the province but not the castle itself yet. If you do hold the castle (London) as well, then you don't have to worry about excommunication. You *will* still have to deal with possible English counterattacks (at least until/unless they sign a ceasefire with you), but that's it.

So if you do control all of Wessex, then you've got nothing to worry about as far as His Holiness is concerned. Also, as I stated in my previous post, you've got 10 years from the time of the Pope's initial warning to war with as many Catholic factions as you wish (aside from the English). Given that, this may be a good time to grab some extra lands elsewhere.