Public Service Announcement for my Non-Brit Friends
If you've kept UK currency for your next trip to Britain, you ought to know that a new plastic five pound note was introduced a few months ago, a new pound coin last week, and new two pound coins and ten pound notes are on the horizon, not sure of the date. The old paper fiver won't be legal tender after May 5th, the old pound after October 10th, and so forth, though British banks will probably let you change them for a while after that, and the Bank of England will always take them. That's assuming that they aren't forgeries, of course, which is the main reason why the coins are being changed!
The bottom line - change the old currency or spend it while you can, because it will be problematic further down the line.