p2p trading newbie question

I was just trying to deposit a small amount in binance with my debit card but it keeps saying "there was a problem.." so I decided to try p2p trading, however now, got a recipient issue ....lets say i found a good offer but he uses transferwise... i want to buy 50usdt in GBP(I'm from the UK ), so I transfer him 50 in GBP but in transferwise i cannot use the same currency as recipient and sender, so that confuses me, which currency do I choose as sender ? I'm in the UK btw