The expert is correct.
This is an old chestnut, and is based on the GBP's confusion between "legal currency" and "legal tender". "Legal tender" is a very specific legal term that only refers to methods legally guaranteed for the payment of debt - it doesn't affect general transactions. No UK&NI banknotes are legal tender other than Bank of England notes, but they can of course all still be used in shops.
As an aside; all UK coins below £1 are restricted as legal tender in that they can only be used for debts below certain amounts (so, for example, you can't pay a £200 bill with pennies...)
[I'm on my mobile ATM so can't post links, but you can read all about this on Wiki.]