Legal tender has a pretty narrow defintion. It basically means what can be used to settle a debt, in the absence of contracted payment terms. Here is a snippet from the Bank of England website:
"So, what counts as legal tender?
It varies throughout the UK. In England and Wales, it is Royal Mint coins and Bank of England notes. In Scotland and Northern Ireland, it is only Royal Mint coins and not banknotes.
There are also some restrictions when using small coins. For example, 1p and 2p coins only count as legal tender for any amount up to 20p. And 5p and 10p coins only count for any amount up to £5. But £1 and £2 coins count as legal tender for any amount."