Have you tried going through a local Post Office as when I renewed my license last year at the PO there were a couple Polish guys ahead of me changing their Polish licences to UK ones, I assume ahead of BX.
It will cost a few quid more but they will put you straight on anything that’s not acceptable in the way of paperwork.
Just one other thing try using a PO that is not over subscribed with customers as there are few near me that haven’t got the time of day to bother with you but there are a few still about who will bend over backwards to help you.