This is splitting hairs. yes there is no legal requirement to accept payment of a debt with them but "legal tender" is widely taken to mean "money accepted in shops"
It's not splitting hairs at all. It's the definition, and it's to do with whether you are entitled to settle a debt with particular currency. If you offer money in the form of legal tender the receiver is not entitled to refuse it. Leaving aside whether it is suspected not genuine, technically if you offer a shopkeeper a Bank of England 50 pound note and they refuse it you can consider the debt settled and you are not obliged to offer anything else.
Scottish banknotes are not legal tender, but they are legal currency. No banknotes are legal tender in Scotland. English Banknotes issued by the Bank of England are legal tender in England and Wales but nowhere else. Coins minted by the royal mint are legal tender everywhere in the UK with the complication that 1p & 2p coins are only legal tender up to the value of 20p.