Depends how old they look i suppose, but you could go out for a meal - which you will all likely get served (try TGI fridays and get cocktails), then your best bet is probably to go to pubs rather than bars/clubs as they are less likely to have bouncers or ID you as soon as you walk in.
Try go to busy pubs and sit away from the bar, then you can get the drinks without your friends going to the bar - this is what me and my friends did while i wasnt 18.
Just make sure you all look confident, lol!!
You could have a fancy dress party? would be memorable! Or a cocktail party at home if you can, then you can all dress up but just have the cocktails at home, then maybe head to a pub for a bit after.