Please feel free to comment on, or tear apart, this post. I'll start with the first criticism, this post is, way, way too long!, sorry.
Ok, I've been searching and searching today, and looked into many of the numerous solutions available, on Amazon (UK), and a bit of ebay.
Thanks for all the replies and banter, so far. It is helping me land on the ground running.
I'm really itching and itching , to get something to start me off.
One plan, is as follows (not necessarily followed, exactly as the link shows):
https://www.instructables.com/WiFi-Security-Camera-With-a-Pi-Zero-2W/N.B. It would just be a starting point, for initial experimentation, and would fairly quickly and easily, get me going, now. At least to some extent.
The plan would be I install them, so they look through suitable windows/door(s), so don't need to be weather proof, would only need a relatively cheap USB power supply, with the built in WiFi, 'connecting' it up, to my network(s).
Although I think I can arrange it, (with some set up time and other initial hassle), to be a separate part of my network(s), a bit like a vlan. With equipment, I already have. But, at this stage, there doesn't seem to be a need for it, as long as we are talking about fairly trustworthy PI software sources, such as the Raspberry PI foundation, etc.
PROs (for me, at least):
Unlike virtually all the other brands, where (like others have said), it would be worrying to directly connect them, to existing network/NAS etc infrastructure. It would be Raspberry PI Zero 2W's, with my own (as in open source PI software, suitable for OS and CCTV). So, reasonably trustworthy, to connect to my existing equipment, without any major creation of isolated network branches (VLANs) etc.
Although, I'd have to check to see if the CCTV camera software itself bit, was truly open source, or just a big binary blob, supplied from some source, which would be less trustworthy.
The 12 megapixels of the Raspberry PI Camera module 3, is actually a respectable pixel amount, as such cameras, seem to go for like the £100 mark, just for each CCTV camera.
Wide angle or normal, and with or without IR filter versions, are available.
There seems to be a Raspberry PI Zero camera case. I need to check if it takes the Raspberry PI zero 2 and Raspberry PI camera module 3. (The article probably indicates it does, but I haven't read it through, properly, yet).
I don't think that camera case is weather proof. But I prefer to start off, quickly and easily, by using those through windows and with WiFi, initially.
It will help me learn what I might need, for a more permanent and probably outdoor camera solution.
I might get some cheap dummy, easy to fit cameras, to give me better deterrent capabilities, outside. Until I'm ready, for a proper external CCTV system.
It is dawning on me, that the permanent, outside solution, with CCTV cameras (IP), involves considerably more work, and equipment/cost, than I originally realized. Each camera would need its own Ethernet cable, and the cable runs back to the main station. Which could be a PoE router, with enough wattage capability, for all the security lights (on the better cameras) to be powered, that can add up to a fair chunk. Cheaper PoE routers, would not be able to handle such output loads, depending on what things you get.
Cameras without subscriptions, also seem to offer rather basic functionality, as regards the software/apps/web-interface support.
As mentioned by many in this thread, already. Modern day auto/motion detection/alerts, doesn't work very reliably at all, usually.
NVR's, can be a mixed blessing. Some, only really work with their own range of cameras. They tend to come with relatively small hard disks (e.g. 1TB, which is enough, I guess, really), and if they come with bigger ones, the price seems to shoot up, rather quickly and steeply.
The cost of all the PoE equipment, Cameras (the more features and bigger resolution ones, start to get very pricey, especially for quality ones, e.g. £240 (I appreciate there are much cheaper options, but if you want high resolution (12MP)), PTZ, quality makes, other features, etc, then you perhaps need an expensive one) per camera), outdoor suitable and suitably long cables (Ethernet), monitor(s), higher end NVRs (if used), and other bits and pieces (such as high endurance SD cards in cameras if applicable, battery backup if wanted, etc), can soon bring the price over £1,000 (which would be a lot or even considerably more than I originally intended) for a setup.
Just to reiterate. The desire for top resolution, doesn't come from greed. It is to provide the Police (if necessary), the best possible images, to track down the criminals, if necessary. Also, enough resolution and quality to make out number plates, could be very useful, as well.
If they are wearing masks, high resolution and very clear images, would still give a better chance of sussing out who the criminals are, and/or recognizing them again.