I am not in favour of widespread, unaccountable surveillance of UK citizens.Don't you think Tempora constitutes widespread, unaccountable surveillance? Also why the UK citizens qualifier?
During my time we disrupted a number of exceptionally serious terrorist operations.[1] [2] are public knowledge and there are a few others that are not open source at this time.
Most people would applaud you for that work and support some level of targeted spying, overseen by the judiciary, that doesn't mean spies should have carte-blanche to spy with impunity on everyone. Judicial scrutiny would make life more difficult when surveilling terrorists, but we have a limited justice system because it is fair and moral, not because it is maximally efficient.
Neither I, nor any of my colleagues, were instructed to keep any files on individuals to be used as political leverage or "for dirt" at a later stage.
If information is kept on everyone, you wouldn't need to be. The information is stored, ready to be used at any point in the future, and your boss doesn't need you to access it. The concept of keeping files seems a bit quaint in an age when massive datacentres are built to store our data, and every communication, no matter how mundane, is being logged for future use.
I would also note that British intelligence and police have on multiple occasions spied on individuals for use as political and economic leverage, and given Tempora, they wouldn't even need to spy on individuals specifically, so your assertion here rings a little hollow.
It's not all about us or some perceived intrusion, it's about power and the very real threat to a democratic and free and open society that universal surveillance and unaccountable spy agencies pose. Also interesting that you bring up the IRA and the attacks of the 90s - that violence wasn't settled by surveillance and repression, but by compromise and self-rule.