That’s a really good point actually @Richard.W, I’ve never thought about that.
I, at the moment, transfer USD from HSBC to revolut (over the free limit) convert to GBP and transfer back to HSBC and even that is cheaper than converting that internally in HSBC.
I also use interactive investor and the 1.5% fee is a thorn in my side. I wrote to them and ask multiple times to allow us to deposit non-gbp currencies and the answer was no. I guess that’s what T212 can use to prevent abuse of this system. We can keep the current no/low fee conversion but can only deposit/withdraw our home currency. (which is opposite of what @Vedran mentioned )