If this policy expanded to all UK banks, it could plausibly make the UK a less interesting target for ransomware and similar cryptocurrency-backed extortion schemes.
I believe Australia is moving towards making it illegal to pay crypto ransoms. Cybercrime has gone absolutely wild over the last few years and cutting off the money tap should do some good.
At least in principal, with a service like Western Union, an actual human or other KYC’d entity needs to collect the money at the other end. If you run a $100M ransomware campaign, a very enthusiastic prosecutor is fairly likely to arrange to arrest people well before $100M gets cashed out, and without perfect opsec, you have a problem.