I live close to London and the drink prices really do vary on what kind of establishment you choose to go to.
I think your best bet is a buffet. The cheapest way to do this, I think, would be to book & pay for a function room in a bar & pay for the buffet which the bar will then arrange. It wouldn't be anything extravagant, but it will feed people and the most important part of the event is to have everybody in the same place, right? Also bars tend to be cheaper for drink, unlike hotels. Hotels and wedding venues seriously rip you off when it comes to drink! I'm for the 'Pre-pay for the venue & buffet, and everybody pays for their own drinks on the night' type deal. That seems like less of a headache to organise, too.
In the UK, I have never heard or been to an open bar with a time limit instead of a money limit. I don't think an open bar is a good idea. Some people drink a little, some people drink a lot, and others might not even drink at all. I don't think it's fair for somebody who doesn't drink to have to contribute to the drink kitty when if they were buying their own drinks their evening would be considerably cheaper.
Also, I'll echo the ID statement. Even though you're legal to drink at 18 here, pretty much everywhere operates a "Look 25" policy and if they think you look under 25, they will ID you. I get asked for ID absolutely everywhere I go, and I'm 24. Typically, they'll only accept a drivers licence or passport, but I'm not sure about overseas drivers licence so perhaps a passport is your best bet.