This is not a duplicate, because obviously you're a citizen, but I would recommend following the complaints process as in this answer. Given you're a citizen I'd also recommend contacting your MP (particularly if you don't get a prompt response to your complaint). travel.stackexchange.com/questions/96671/…

@MJeffryes thanks for sharing that question, I've learnt a lot about the process by reading that.

I'm a U.S. citizen I'm on a similar list in the U.S.. Until it gets resolved—as I certainly hope it does—try to be as polite and as patient as you can with the people you talk to. They're probably doing behavioral profiling to see if their questions make you nervous. And at the end of the day, they're just doing their jobs like anyone else.

Not an answer and not nearly as bad as you, but two anecdotes: 1) I reported my old passport as lost and since then on my new passport whenever I enter the EU in some countries they spend an extra 5-10 minutes manually checking stuff after refusing entry through any automatic gates. 2) My wife's passport from central europe somehow gets an incredible amount of attention in all central asian nations. We have absolutely no idea why, and they always let her through in the end, but something about her passport is apparently suspicious 🤷‍♀️ . Point is: These things can be pretty weird.

Would a Freedom of Information request be worth pursuing? I'm guessing there are some exceptions to the FoI act for law enforcement, but it might be worth considering.

I agree with @adam.baker: "begrudging" is most definitely not the right attitude in this context. I had a friend who, passing through a UK port, was asked about one of his cases (possibly for ski poles) and made an ill-advised "submachinegun in violin case" joke: he was taken to one side by large humourless gentlemen and despite normally being chatty would never disclose what transpired over the next few hours.

@MarkMorganLloyd There is, luckily, a not-so-fine line between a begrudging attitude and downright idiotic behaviour, such at joking about having weaponry in your luggage at an (air)port checkpoint.

@JanusBahsJacquet well /yes/, but the point I'm making is that these people have (a) considerable incentive to not let a tsarist agitator (or whoever) through on their watch and (b) considerable latitude in the way they work to make sure it doesn't happen. You quite simply don't want to upset them when they're bored, and you even less want to upset them when they're stressed.

@Mark Definitely. But ‘begrudgingly’ is quite a broad spectrum of things – it’s ‘begrudging’ to give them the info they ask for while letting them know that you find it unnecessary and annoying to go through the same rigmarole every time you visit the country you’re a citizen and passport holder of. But if you tell them politely (acknowledging that it’s some system’s fault, not the individual officer’s), their latitude doesn’t reasonably extend to escalating the matter to detaining you (and if they do, there are avenues of complaint available after the fact).

As they seem so surprised that you live abroad: If you have been on the electoral roll before you left, you should be able to contact your local Electoral Registration Office to see if you are still on it at an obsolete address, or you could try and register your address abroad (gov.uk/register-to-vote). This will only work if you have left the UK less than 15 years ago.

@AlexHayward It will not because anything that would lead to this situation for OP will be covered by national security exemptions from disclosure.

This isn't really a time-saving solution but you could skip UK immigration entirely if you fly into the UK via Ireland.

How old is your passport and how representative is the photo? A few years ago my appearance changed somewhat (long hair, beard, spectacles) and my old passport photo really didn't look like me. I renewed my driving licence with a new photo and this led to the automatic gates rejecting me every time and the humans giving me extra scrutiny, eg questioning, asking for other forms of ID (no form filling though). Everything was fine after I renewed the passport with an up to date photo.

@MarkMorganLloyd they can't "not let a tsarist agitator or whomever on their watch" if that person is a British citizen. Furthermore if a British citizen refused to answer questions that they have no right to ask after explaining that he is sick of being delayed at the border, they might well be sympathetic.

@phoog with respect, the friend I mentioned was English, British born and bred, the proprietor of a significant company, and while not ostentatious did little to conceal his family's prosperity. He and his family were on their way out of the country, via an English port, on a skiing trip. And that didn't prevent his being given a very tough time for several hours. I don't know where you are, but to spell it out: "The Excise" (a name which should be familiar even to an American), and their derivatives and associates, don't need things like warrants.

@MarkMorganLloyd An educated guess would say the humourless gentlemen would look for arms everywhere on and in your friend. Not a smart move.

"I am a UK citizen and full passport holder since birth." Just to be clear, you were born in the UK to atleast one parent who was either a permanent resident or themselves a UK citizen? I ask because if this was not the case, there are scenarios where there may indeed be questions around verifying your citizenship. And it may not be obvious that this is what they are doing - meaning it is not as simple as just verifying your identity which might be a relatively trivial exercise.

And I suppose I should note that even being born in the UK and later obtaining a UK passport (you obviously weren't born with it in hand), doesn't necessarily mean the matter is settled. There are some interesting cases of surprised brits and you might be close enough to a borderline case for them to scrutinize the details every time you use your passport.

This might be too much to ask, but is your place of residence some "hotspot" like Syria, Libya or Palestine?