Does it make sense to open a USD fx account in the UK?

I am planning to open an fx NDD trading account in the UK, but I want the base currency to be USD. That is because I want the money to stay in USD when I do not have any open positions.

Is this a stupid thing to do?
Do most UK people open GBP accounts as opposed to USD?

The fx broker representative, whom I talked to, tried to convince me to open a GBP account instead, he said I would be hit by conversion rates when depositing and withdrawing funds otherwise. Can I circumvent this if I open a USD account with my bank in the UK? Overall, does it really not make sense to open a USD account given that I am in the UK?

Thank you for taking the time to read. I will be greatfeul for any advice.