another question is how can i even trade USD - GBP ore EUR - GBP on exchange ?
it does not allow me to deposit GBP as currency
is there another way to change currency to currency on exchange ?
For a start, what exchange or exchanges are you trying to use?
You can trade USD - GBP or EUR - GBP mostly in forex exchanges. In crypto exchanges like Binance USD is represented by stable coins such as USDT, FDUSD,USDC. You can exchange your USD for the stable coins and then use them to trade Bitcoins and other assets such as BTC/USDT, ETH/USDC
If you want to cash out, you can sell your BTC, USDT or USDC for USD on their customer to customer marketplace.
For EUR and GBP, they have no stable coins on Binance for example, so you can directly buy and sell Bitcoin using EUR and GBP as in BTC/EUR or BTC/GBP.