Currency conversion orders can only be executed with the funds resting in the Securities segment of your Interactive Brokers Universal account. If you have an approved UK Regulated account segment and trade any CFDs, OTC Metals or Non-US Index options, then the cash in that UK Regulated account segment must be manually transferred by the account holder back to the Securities segment in order to convert that cash to a different currency. Please remember that the "Total" cash amounts for each currency displayed in your trading screen are the net amounts between all of your account segments: Securities account, Commodities account and UK Regulated account.
If you have closed all of your positions in the UK Regulated account and the cash is settled, please see How to transfer funds out of a UK Regulated account.