I don't think I've ever gone to a restaurant expecting not to pay without being told in advance, one possible occasion was a celebration meal at the end of a project, but even then I made sure I had cash on me just in case. Every birthday meal I've been to I've paid, maybe I just don't know people rich enough to host 20-30 people at £30 a head just for a birthday.
As for the whole splitting the bill thing in general, am I the only person who is completely neutral about the whole issue? If people want to split the bill evenly, that's fine with me. If people want to divide it up by what people had, that's equally fine with me.
As I nearly always drink alcohol with a meal and know quite a few people who don't, I always offer to pay slightly more than my share to subsidise the non-drinkers.