There is no UK restriction on transfers. I have happily moved from EuroCheat BA ac to UK Avios account. If the transfer fails it is because Avios does not have your DOB or it has one which does not match your BAEC account. DOB is the key security match. Annoyingly, though, you cannot see your DOB on the Avios site, you need to call. And they need a passport copy to change it if incorrect.
Or, of course, your Avios ac is a HHA. No restriction on BA HHA, I think.