Trading pairs are only available when you trade on certain exchanges, but when you want to move coins you will see for yourself whether it is in a Bitcoin or GBP address. This is not a case of combining BTC and GBP as there are no address options that combine such a choice. BTC yes with a BTC address but otherwise GBP depending on what network provides the GBP coin service. Stable coins are usually ERC20, BNB, or TRC20 depending on choice. So do you understand correctly that a pair of coins does not mean a combination of one coin?