Many members have already given the answers to your main query, and that would be if you want to transfer BTC from the first exchange you need the receiver address and you can do it quickly, but if you want to sell BTC on the other (2nd exchange) and wants GBP in return then make sure that the BTC/GBP pair is available on the exchange you have selected.

And talking about this question you first have to find an exchange where you can exchange or trade in forex, this is a forex exchange where you can trade in dollars or any other currencies which are in fiat form. After finding the forex exchange you can easily trade with your specific pair.

Yes, the pair you select can help you to change your specific currency like BTC into another currency like USDT and the pair will look like BTC/USDT.