I have a feeling that getting something in my UK immigration data deleted is about as easy as walking through a brick wall, but maybe I'm wrong. Has anybody ever tried?
I have a UK passport. About 8 years ago, I left the UK by ferry and the departure was not recorded. When I returned by plane to Stansted, I was kept waiting for about 20 minutes while the immigration officer checked this "irregularity", then he let me enter. It seems he wrote something in my record like "arrived in UK without apparently having left UK". When I arrive in the UK, I can see that they notice this, but the official just taps a couple more keys, presumably to see if I'm in the police database, and then lets me in. But when I took a train from Hungary (EU) to Ukraine (non-EU), I was flagged by the Hungarian officer with a hand-held computer, made to sit in the immigration police office for about an hour while they checked, and when they let me go, of course the train had left (and there aren't many trains or buses a day in those parts). So presumably the Schengen Information System is linked to UK immigration. Last time I was in Ukraine, it would have been nice to take the bus to Slovakia, but it wouldn't have been so nice to sit in the Slovak police office for an hour, so of course I didn't take the chance.
Thanks for the comments, everyone. 1. AFAIK, eight years ago, the ferry companies sent in passenger lists to the authorities for each trip. I arrived at the ferry at the very last minute, presumably after they'd made up or sent off their list. 2. Surely the information held has to be an exact match and include the passport number? Otherwise if someone with a common name like John Taylor is wanted for murder, there will be several other blokes with that name and the same date of birth who will get flagged and maybe even detained. 3. Of course I can't be 100% certain that the incident in Hungary was related, but I've travelled all over the world and never had any problems with immigration or law enforcement anywhere. 4. I'd be interested to know if something similar has happened to other people. There must be others who "arrive in the UK without having left the UK", such as those who have their own boat, people with dual nationality, etc.