Except your Thai born wife was doing these changes in Thailand. The procedures for the locals can be cut and dried but any similar dealings by a foreigner is fraught with the incorrect translation, letter from embassy or certified by MOFA circus.
Don't the UK authorities issue some formal documentation with the before/after details of the name change by deed poll? I recall reading a fairly recent post about someone seeking advice on what's needed for Thai immigration to transfer/renew an extension when the same person is presenting their new passport with their new name (deed poll change). I recall mention of there being two official methods of doing the name change in the UK but only one provides this supporting documentation?
If the OP's friend still has full control over the existing, old-name accounts, including online access, why not open new account(s) and do regular transfers from old to new accounts over a period of time?