There are machines that will let you tip in bags full of coins in return for a voucher. I believe there's one at the supermarket near here. I understand that you get the full value for the coins as a voucher. Obviously that's only useful if you shop wherever the voucher is valid but it's worth looking for such a machine at your local supermarkets.

Any old coins would be worth keeping or selling to a collector. You should also check any recent 20p coins that you have to see if they show the year - I found one recently that didn't and these coins are worth £40+ to collectors.