They aren't "legal tender" but, as it happens, that doesn't matter because they are legal in the sense of legally usable as money in the UK. The term "legal tender" has a very specific, and mostly irrelevant, meaning.
Our money is legal tender but they won't accept Manx notes in the UK, it has to be changed into English first before visiting.
You should all feel very proud (imho)
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oh but on the other hand they don't want it do they... and "we won the referendum, so there". It would rely on thousands of London civil servants to progress the negotiations and on Westminster to enact a mountain of legislation to make it all happen.