Bank of Ireland Banknotes legal in england?

Today at work i got a tip. The man gave me a 'Bank of Ireland, �5 Sterling note' as he didn't have anything smaller then a �20 in normal money. He said it was legal tender and i though it was too. However i've been to 2 different shops and neither of them would accept it. Is it not legal tender, and if it is, are the shop keepers allowed to refuse it? thanks