There is no currency exchange fee, but there is a small sell and buy spread. So you do lose a tiny bit on each fx exchange compared to that instant’s average between buy and sell spot rates. All other things being equal (no change in prices or fx rate) a round trip GBP-USD-GBP seems to cost about 0.03%, as close as I have been able to figure. That is based of something like sell and buy 1 GBP for USD 1.24000 and 1.24015, using target spread of 0.00015 on the CFD platform.
The nearest competitor amongst other brokers I know charges 0.1% each way, or 0.02% + 10 euros each way (Degiro). That will be on top of sell and buy price difference effect. The worst I have seen is 1.5% each way with Interactive Investor - however, they allow one to hold both a GBP and USD account, and then make fx exchange infrequently, or transfer out in USD to one’s other USD bank account where exchange to GBP can be done more cheaply (with Revolut). However, they only take GBP on the way in, so the 1.5% must be paid in one direction.
In previous answers we have been told that Bloomberg data is used by Trading 212 but that these rates are not free for Trading 212 to feed to customers. To estimate the rates used for Invest transactions we are advised to look at the sell and buy prices for USD/GBP on the CFD platform. However, the Bloomberg rates have tighter spread, so the true round trip cost is likely less than 0.03%.
Perhaps @George can give a like if I have correctly summarised the situation.