ID required for entry to bars/clubs?

I am just under a month away from my 18th birthday, and in the middle of post A-levels party week. I don't have any fake ID, and I am aware that it is illegal anyway, however I know that several of the parties in clubs/bars may require ID for entry, which is obviously a potential problem. I would not expect to get served without ID inside, and a lot of establishments know it is easy for an over 18 to buy a drink for someone else and so are wary of serving more than one drink to one person, and go around to check IDs, not just at the bar itself etc. I am not particularly bothered about not being able to drink, I can have fun without alcohol, but it's obviously annoying not to be able to even get in, and I'm wondering what the likelihood of being turned away at the door is? Are they very strict on ID for entry, as much as they are for ID at the bar, or are they usually ok with it?