Bit beyond my knowledge area but I believe with a lot of the large exchanges at present, you never technically trade in direct USD/GBP. Even though it appears you do.
They show it as USDT for example.(AKA, Tether.) You can still withdraw it as USD/GBP etc, it's just they covert the tether to money at the stage you do that. The user doesn't often know they've even been part of a Tether trade. They just see the $ or £ amounts in their account.
That's how it was when I used to trade on Kraken anyway. You were always technically trading BTC to USDT, not directly to cash.
It did mean that if tether was infact all dodgy as a lot of reports say, a lot of the exchanges would crash, as there would be no value in the tether to cash conversion. Therefore destroying the price of BTC in an disastrous fashion.