Travel light, be a weight weeny, I saw a few people carrying rucksacs with them when there really wasn't any need to, there are ample opportunities along the route to fetch food and liquids although carry as much of your own supply as you can on the ride and eat often even when you dont think you need to. Personally in events like this I dont want to stop and I am able to stuff enough gels, oatmeal cakes and jelly beans into my pockets along with the spares inner tubes, levers, pump and mini tool. On my bike i carry 2x1 ltr bottles. This seems to be enough for me although i do hydrate plenty 24hrs before the day. That said, if its a scorcher then do stop and take on the liquid because if you dont you will get cramp and end up unable to finish the ride. I never bother with a lock either, when i used the loo at the start after i handed my bag in i just removed my garmin and the front wheel and pointed Percy at the porcelain. Be careful in those portaloos when you are wearing your cleats, its like an ice rink and i saw one poor chap take a fall down the steps last year and render himself out for the ride. I would also add that making sure that your bike is set up just how you want it and are comfortable on because if that seat post is too high or too low by just an inch you will feel it after 100 miles.
Doing 100 miles is also a bit of a mind game. You've trained for weeks, you're fit and you have put in the miles but the one thing I never experienced before was the isolation i felt after 80 miles when i did the first Prudential London-Surrey ride. I have done several 100 mile rides with friends but the previous 2 Prudential rides i have done by myself. At first you will be in groups and chain gangs flying along at speeds that you would not normally average. Eventually the miles and the hill climbs take their toll and I found that the groups around me started to break up. At around 80 miles I was pretty much riding alone which was fine but compared to the previous miles where i was in various groups, having some banter, drafting, taking the front, basically keeping myself occupied which made the miles fly by, it was now becoming a slog and that wasn't good. Eventually a few of us who were riding at a similar pace managed to group together for the last 10 mile run into London. This did wonders for our spirits and we breezed into London on a high. So I guess all i am trying to say is be prepared for that. Oh, and that sharp climb in Wimbledon towards the end is a right *word censored* yet few people seem to mention that....
Have fun and enjoy it, its a blast.