Actually, yes, I think what Mandy2003 is saying is worth noting. Make sure you know how to get out of London on foot/by tube/by overground. Could all get a bit haywire.
We got tickets to Live8 and knowing how to get to the nearest overground (as opposed to tube) was a godsend when it all over-ran, tipped into end of tube time, stations were shut, etc.
Obviously the wedding won't run that late, but tube stations do have an annoying habit of being shut when over-crowded, etc. If you know where you can walk to it really helps.
In fact, last London trip for us was to Bethnal Green. Someone vandalised a tube train and so the whole central suddenly line shut down. Bus stops were therefore mobbed. We walked to Bethnal Green overground and were the only people on the platform and in Liverpool Street within minutes! It was a ten minute walk, I think, down a funny little side street but way better than any of the alternatives. And you can get anywhere from Liverpool Street.
Likewise, for the Queen's Jubilee we got a train into City Thameslink and were again almost alone despite the heaving crowds outside. (In fact, I think there were even toilets there....).
Worth a bit of map and journey printing from tfl.gov.uk in advance of the big day....