@TwoMonthsOff
Again, the extent of covert surveillance' available to Capita reps would be hoping they can spot your TV showing BBC programming through your window as they walk up your garden path, or perhaps surreptitiously listening through your letter box in the hope of hearing the Eastenders or BBC News themes.
None of that would be of any real consequence in any case, because it's still entirely at your discretion whether you choose to acknowledge their knock on your door or not, and no magistrate/sheriff is going to issue them a warrant without categorical proof your TV set was on and showing BBC, which would likely involve a timestamped, verifiable photograph, which they have no right to take through your window, or a recording of your own admission.
It really is as simple as 'ignore them and they cease to exist', because they are utterly powerless without some degree of cooperation and self-incrimination on the part of the viewer.
The vast bulk of fines are levied on unwitting homeowners who allow Capita reps into their homes, usually after a barrage of misleading or not entirely truthful statements and assertions. One of the reasons women are disproportionately affected is because they will often call during the day, find SAHM and single parents busy looking after small children, and sweet talk them into allowing access for a 'check'. Of course, if the TV is on and showing BBC, they have an instant winner, but even then it's usually a case of offering them an on the spot sign-up for a licence. It's not everyone who has the funds available to just acquiesce to that though.
It really is a despicable and patently dishonest way to conduct a business. Which other company is allowed to harass and threaten you with impunity because you are NOT a customer? And yes, they are a business, and they use stationery and language that intentionally carries implied threats. They mail indiscriminately and regularly, and they often ignore demands to cease, so I would contend that their typical practice absolutely does constitute harassment and threatening behaviour.