@ Cowardly Anonymous
"Love it when the schoolyard lawyers start yapping.
You can indeed be arrested for trespass. Been there done that. It can be a serious criminal offence depending on your behaviour."
Schoolyard lawyers yapping? Hardly. Anyway:
OK, you can be arrested for aggravated trespass (caused criminal damage on the way in, hit the landowner, etc) or criminal trespass (British rail land where its designated active, MoD land, etc), but (as this thread is clearly alluding to) walking into someone's garden and using their pool, outwith PACE or Public Order 1986, is a civil offence in its own right and to the letter of the law, that is not criminal therefore not arrestable. You might of course be arrested on 'suspicion of breaching the peace' but thats another matter entirely.
Mine's the one with 'graduated from Cambridge with a law degree, spent 7 years trespassing and never got arrested' written on the back.