New riddle, and also, I live in the UK... so...
Three people go into a restaurant and order a meal, the bill comes to £25. They each give £10, the waiter brings back £5 change. They each take back £1, and leave the remaining £2 for the waiter as a tip.
So, they gave £10 and got £1 back, which means they spent £9 each. OK £9 x 3= £27 + £2 (for the waiter) = £29. What happened to the missing pound ?