Since the middle of 2018 those passing the RSGB exam for the Full licence have been able to apply for short call signs such as M5xx, this has provided a major incentive to upgrade to Full.

“An applicant for a new Full Licence (or a Full (Club) Licence) may request a call sign with three trailing letters, using any valid Full Licence prefix in the list, above, except ‘G2’. Call signs using formerly issued prefixes are not available via the licensing portal and must be requested on a paper application form (a fee is payable).

A callsign with only two trailing letters or which starts with ‘G2’ is only available if the applicant previously held it. Our policy that a station may change its call sign only in exceptional circumstances remains unchanged though Ofcom may, on occasion, require a station to change its call sign.”

— Ofcom Announcement

The change in policy affects the issue of old amateur call signs that had been issued prior to the start of World War 2 but it is unclear why M5xx calls are also affected. Up until mid-2018, this call block had never been issued before.

The new policy does not affect any call signs that have already been issued.

Ofcom Statement "Amateur radio callsign allocation" - https://www.ofcom.org.uk/manage-your-licence/radiocommunication-licences/amateur-radio/amateur-radio-info