Legal Tender Guidelines

This I never knew:

Coins are legal tender throughout the United Kingdom for the following amount:

£5 - for any amount

£2 - for any amount

£1 - for any amount

50p - for any amount not exceeding £10

25p - for any amount not exceeding £10

20p - for any amount not exceeding £10

10p - for any amount not exceeding £5

5p - for any amount not exceeding £5

2p - for any amount not exceeding 20p

1p - for any amount not exceeding 20p

So basically shopkeepers aren’t even obliged to let you buy a packet of crisps using all 1 and 2p pieces. Wikipedia (obvs) has more about the 25p coin and the Mint have a cool gallery of £5 coins.