You've just hit the nail on the head with your own logic. To see things from a long way away you need to zoom in, if you zoom in you can only see that tiny point in space you are looking at.
That is great for a PTZ camera if you want to focus on a specific area, the main pitfall of a PTZ is that if its focused in on one area, its not looking at anything else in the same area it is meant to be covering when zoomed out and you create a massive hole in your coverage as a result.
That's not so great if you need to maintain coverage of a massive perimeter fence. So in reality you are going to need hundreds of relatively wide camera fixed position cameras covering the whole perimeter to identify targets for your PTZ's to look at if you want PTZ's.
If you've got full coverage with fixed position cameras, you don't really need PTZ's anyway. PTZ's can be a security risk in their own right as they can be moved by an operator (who will probably be a low paid civilian) to literally 'look the other way'.
In any case, I never even said its the cameras that are the expensive part, because they aren't. All I said was it's not cheap and likely to run into 7 figures for a single base. Scale that up across the MOD estate and all of a sudden you have a rather large number.
The expensive parts are all the bits a human has to touch such as installing all the infrastructure needed for said cameras, the maintenance and the ongoing monitoring. Just having a single person monitoring all the cameras 24/7 gets you to 7 figures over a 7 year period anyway, as in practice, having a single person on watch 24/7, 365 means you need to employ at least 5 people to cover it full time and even then 5 isn't really viable as you have zero redundancy. So really you are probably talking about 7 figures in 5 years, just to have a single person sit there and watch the cameras.
The whole conversation is elementary anyway because there isn't any guarantee that CCTV would have even prevented it int he first place. Even if they were spotted you still need a patrol to get there and intercept them before they make it to a plane, it's kind of redundant if they don't make it to the target before they, you might catch them but the damage is done.