Having sold hundreds of .uk domains experience has taught me to wait until transfer to new Regsitrant has been processed, before doing the Tag change. I am happy to update the nameservers upon receipt of payment.
Most people play ball and things run smoothly but it is a pain in the arse chasing folks who don't complete the transfer paperwork and send it to Nominet. Worse still are the minority who never intended to do a transfer. If you need Nominet's help to reverse a Tag change it will cost you £15.