Converting USD to GBP while i'm in the UK

Depends on the amount you’re transferring involved.

USD<–International transfer–>USD<–Conversion via Revolut–>GBP under certain circumstances may be significantly cheaper due to Revolut exchanging it at the spot interbank exchange rate (*Weekend surcharge, fee-free quota for non-Premium monthly transfers.), whereas legacy banks may give an uncompetitive FX rate + charge a commission on top of that, but it’s context-specific.

It’s a SWIFT thing. International transfers in the legacy world are, for some reason, quite expensive.