So which one?
IBKR have a Swiss CH-IBAN account number, so standard rates for domestic bank transfers should apply.
…except for the super crafty banks that will charge you a foreign transfer to Citibank London, despite going to a CH… IBAN. Wasn‘t Migros Bank doing that? But 99% percent of cases, it should be free.
First withdrawal in a month at IBKR is free, too - though I don’t know if IBKR U.K. will let you withdraw USD to a U.S. bank account using a domestic transfer.