You bought BTC with GBP on 1 exchange and now you have BTC in your hands and what you will be sent from 1 exchange to another will be BTC and the 2nd exchange where you are receiving the BTC is not supporting GBP instead they are offering USDT or other pairs. Which means you would sell your BTC for USDT but not for GBP.

If you want to find a GBP pair then try to find a 2nd exchange that supports this pair. There might be other ways to convert the USDT to GBP but it depends on the 2nd exchange you are using. The simplest way to explain might be like this BTC->USDT->GBP

And if you want help with how to send BTC from 1 exchange to another then you should click on the BTC asset on your wallet and there you will find the withdrawal option, after clicking it you will be asked for the network, recipient address, and the amount you want to withdrawals, most of the exchanges detect the network type when the recipient address is given and the fee for the transaction is also provided at the bottom. It is different for almost every exchange.