This question is somewhat related to: Paying debts in cash in a petty way
The Royal Mint issues commemorative coins. For example, they issued £20 coins (made out of silver) but usually shops and banks don't accept them. They are legal tender, however.
If I was sued and had to pay a debt to someone in court, could I use my commemorative coins to pay off part of the debt?
The Royal Mint website says this:
Legal tender has a very narrow and technical meaning in the settlement of debts. It means that a debtor cannot successfully be sued for non-payment if he pays into court in legal tender. It does not mean that any ordinary transaction has to take place in legal tender or only within the amount denominated by the legislation. Both parties are free to agree to accept any form of payment whether legal tender or otherwise according to their wishes. In order to comply with the very strict rules governing an actual legal tender it is necessary, for example, actually to offer the exact amount due because no change can be demanded.