You're already organising the batchelor party for next May? This might be a bit lenghty, but as I live in London and have my own batchelor party coming up in about 3 weeks I might be of some assistanc. No idea what's happening for mine though as it's all in the hands of the best man. It is likely to be in London though so if any good tips arise I'll pass them on.
If you're going for a straight razor shave, I went to Trumpers a couple of years ago. One bad point is that, at least when I was there, they had the ugliest suspended ceiling you could imagine. Discoloured panels, protruding wires and everything, which when you're staring directly upwards for the duration is not the most pleasant of views. Also it wasn't that much of an experience as I was attended to by the most terribly efficient old man who had me shaved in the space of a couple of minutes which left me tending my (free) single malt and looking through their magazines.
Other places for shaving might be Murdock's in the basement at Liberty, I believe there's a place in Selfridges and Truefitt and Hill in St James.
Places I might recommend are:
Restaurants: Great Queen Street in Holborn (gastropub), The Harwood Arms in Fulham (gastropub again), Hawksmoor (for steaks), Rules (while a bit expensive for the quality of food it's a good setting for a manly meal for men and I hear good things about the bar upstairs). Also, by May Heston Blumenthal's new place at the Mandarin Oriental should be open.
Accommodation-wise you're probably going to want to be in or near the West End. You might get a cheaper room staying further out, but you may well end up paying more when you factor in how much you'll spend on taxis. Not overly familiar with the hotels due to living here, but I know the various areas well enough.
Another thing you might try, is a visit to Sipsmiths which is a small independent distillery in London. You go there, try various vodkas and gins and make cocktails. The people running it are apparently a good laugh and it's a good night out (unless, like my best man you have to be at work the next day).