I’m busy with the process currently.
I hold an ICAO ATPL. If you have a multi-pilot type rating and 500 hours on multi-pilot aircraft, you qualify for an exemption from the ground school and can simply book exams and write. Unfortunately no exemption from the 13 exams.
Once you’ve completed the exams, you need a UKCAA Class 1 medical and the flight test can be conducted in a simulator for which you have the 500 Multi-Pilot hours, you are exempt from having to complete any “training” through an ATO.