I have gone through the same. I had a JAA ATPL (UK Issued) which expired in 2016. I maintained my Hong Kong ATPL in the intervening years. Before arriving back in the UK I was told by the CAA that I fall under FCL740 so had to get my Hong Kong ATPL certified by a legal professional and keep the medical and IR current during this time which for me expire next year, and then submit the third country rating application to the CAA SRG 2142. This took around one month to which I received an email informing me I had 6 months to complete the steps to revalidate my UK issued ATPL. As has already been mentioned I set up the CAA portal and completed my initial class one medical again whilst at the same time completed a LST on the 747 which is my current type, and received a course completion certificate from the ATO I used. Once my medical is returned I will then complete forms SRG 1104 (there is a section in this where you tick the expired JAR licence section) and SRG 1308 pay the appropriate fees and wait, the time the CAA take to process is anyones guess, but I believe ok if you put the application in before the last day of 2022. Anyone who has any feedback I would be grateful to hear.
The Cav