I’m glad my research and explanation is helpful to you, and to see that T212 staff have given my post their likes.
I’m not sure what you mean about exchanging GBP to USD before transferring to T212. I thought that it was only possible to deposit into T212 in the base currency of your account. If your base currency in T212 is GBP then deposit must be made in GBP, no?
I realised I can also test the exchange rate using the practice account and information in the History. For buy and then immediately sell of USD-denominated SUWS ETF, I just now got USD/GBP rates of 1.23667 and 1.23682. This is spread of 0.012%, i.e. 1.2 per 10,000. That looks very good to me.