Legal Tender ? Debt?
So, this may not at all be correct but, I was under the impression that "paying for stuff in a shop" is different from "settling a debt".
When you stand at the till and they say "that'll be £11.63 please" then there are limits on what they are required to accept (which, as noted in comments above, are actually surprisingly small so often bigger shops will be more lenient) so you can't give them 1163 pennies and then shout "you're discriminating against me" or whatever if they refuse. This isn't "settling a debt" because you don't owe anything because they haven't given you the goods yet.
But in a restaurant, for example, you are "settling a debt" because at the end of the meal they have given you the goods and so you do now owe them recompense for that. I and thought, perhaps wrongly, that there was a different set of rules about what they were required to accept in payment of that debt. i.e. that they can't refuse to accept because you want to pay a £5.10 bill with 51x10p, for example, because it is actual money of the correct amount and you are legitimately attempting to settle the debt.
Of course all of this is covered by the general observation that life would be so much easier if everyone on both sides of these transactions just resolved to "not be a dick about things".
Which reminds me of the time I donated several years (and Kgs) collection of coppers to the work charity collection and said "You do have one of those coin counting machines don't you?" to which the response was "Yes. But's it's broken. But that's OK because we've got an intern!".