At least in the US a lot of the surveillance is probably illegal and cannot be used in court. They way they get around this is parallel construction. [1] Reuters had an awesome article about this a long time ago, which seems to have been removed from their site. [2] The NSA or whoever spies on somebody they're not supposed to be spying on. They see he's e.g. selling drugs, and further spying shows the time of a sell. They pass this onto a local law enforcement agency who then finds some sort of plausible justification to go stop/search the involved car. A cop, who just happens to have a drug sniffing dog, stops the car for 'driving erratically' or whatever.
If the intelligence agency revealed they were involved then not only could the person involved sue to get his own charges dismissed, but more importantly he could also sue the NSA to try to get the entire program scrapped. Countless entities (Wikimedia, EFF, and others) have tried to sue the NSA for this but it always ends the same way. They can't prove they were hurt by spying, or even that they were spied on, so the cases get tossed for lack of standing.
So they are actually being honest when they say they don't want to give away their capabilities, but that's because what they're doing is probably illegal. At least in the US, but I assume the UK must have something akin to the 4th amendment. To not have a government randomly spying on everybody is one of the foundations of a Free society. We were supposed to learn from KGB, Stasi, and so on. And maybe we did, but not the right lessons.
---
[1] - https://en.wikipedia.org/wiki/Parallel_construction
[2] - https://web.archive.org/web/20130806082051/http://www.reuter...