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.

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