Cheapest in & out USD method?

I have quite a bit of cash USD in Fidelity from my previous job. My Revolut & TW accounts both have GBP as the primary account. I guess someone here also has GBP accounts & tops up USD to T212 so wondering what kind of fees might be likely on e.g. $10k? There’s an ETF I like that’s only available in USD so I was thinking to move it to T212 cash & gradually to that but if fees are likely high then I might use the Revolut premium free trial and convert to GBP to top up that way as usual

Not following every detail of what you’re asking, but

You can hold several currencies at once in your T212 account. You can directly deposit $US from a $US account.

So you might deposit £10000k, and change £5k to $US. When you buy instruments which are in $US, the platform defaults to the currency of the instrument IF YOU SET IT TO DO THAT, assuming you have sufficient funds pre-converted.
Therefore there’s no currency conversion on a buy & sell round-trip. There WOULD be, if you dealt something in say Canadian Dollars without having pre-converted some money to that currency.
You can convert $US to $Canadian, or Euros, say.
You pay a fee when you convert the £5k to greenbacks, but it’s low, Interbank plus 0.15%.

So it sounds like you can deposit your $US, and never use any other currency.

CFD is different. It’s all in £GB, but
There’s no currency exchange needed, until the position is closed. Then your winnings or losses are converted, at 0.5% fee, iirc.

So you deal $1,000,000 worth of Tesla, and you win $2. You pay $0.01 in fees.

It’s more that I already have $10k elsewhere that I’m wondering what fees might be like with TW/Revolut if transferring USD when my TW/Revolut is GBP as the primary currency. So the whole flow would stay USD currency

This, then. Simple as…

But isn’t there some fees to transfer from Revolut to T212 in USD when the main Revolut account is GBP? Card payment would be free but only for a small amount