Interesting. I can imagine it being that way in the US, but are you certain that that's the way it is in the UK? Just curious. I'm pretty sure (but not certain) i've read of similar cases to this here where the guy wasn't prosecuted. (Siimlar to your example with the fake ID from memory. In a bar)
In fact, a friend (of a friend) who was about 27 slept with a 14 year old girl he met on a park bench about after a night of drinking about a year ago who he believed to be 16 (and she admitted telling him that to the police) and he certainly didn't go to jail! (although I don't know what, if any, punishment he DID get)
Another that springs to mind is a French professional footballer who also slept with, I think, a 14 year old and he was like 30 and he didn't get sent down, so I guess other factors must come into play, maybe moreso in Europe than the US