Much appreciated! I read for 30 minutes including a lot of links, and quit. The stated purpose of PNR collection is for the prevention, detection, investigation and prosecution of terrorism and serious crime - drugs, trafficking, child sexual exploutation and the like. I don't think an OAP claiming to live in one place but actually living in another falls into that category. However, the data is there. But whether it would be legal for say the DWP to access it for checking on a pensioner is highly dubious. It's an EU based law, but the UK seems to have signed up to it. As have the US, Canada and Australia. So they are all sharing PNR, flight details, destination, luggage weight, check in times, ticket numbers and a ton of other data. Beyond that, who knows?
When I asked the Philippines IO if the UK government could track my movements in and out of Philippines, she simply said it would be possible, but highly unlikely. So there you have it. YMMV as I always say. Or in Thai "Up to you!".