Wow,being from across the Pond, I can only relate what we are allowed. US currency can not be denied for any payment. Recently a fellow paid his child support, $800, in pennies, which he dumped on his ex's lawn. Classy, no, but legal, yes. I remeber when I was young, my brother lost a bet to my father, which he paid in pennies. he thought he was being snarky, but my father loved it because he always needed pennies for parking meters in town. Rather backfired upon my brother.
Here shops often try to require a minimum purchase amount to use a debit or charge card. Illegal, at least in Maine. If I find myself in a snarky shop, I'll try to buy a candy bar. Needless to say, I have caused some havoc.
I guess it boils down to whether the Pound is really legal tender for all debits, ect. Good question for your Treasury.