Replacing rear springs isn't too hard, and you should replace the shocks as well. The shock's job is to prevent the spring from reciprocating multiple times after you hit a bump, which is what causes the bouncy ride. Some people do this without spring compressors. Get the rear end on jackstands and use your jack to support the lower control arm. Unbolt the control arm from the trailing arm and then let the jack down to relax the spring. Swap in the new one and jack it back up till you can bolt it back together.
It's not too bad. The link you posted is for coil-overs. Those are adjustable, and people like them because they can lower their cars with the spanner wrenches. If you car is just a daily driver, you can buy plain old stock aftermarket shocks. Also, I think you said the front is already done, so you don't need those struts that come with the kit. I don't know where in the UK is the best source of parts, but shocks are very affordable, even good ones.
It's hard to say. The lower control arm has rubber bushings that are probably worn out, and you could replace those if you wanted. I never worried about it, but admittedly my car bounces around due to all the dry-rot. The upper spring seat is actually an integral part of the rear subframe, so you don't replace that. I don't recall that there's anything else in there. Just the springs themselves.
You need the bottom link. The top one is mislabeled by the seller. It's a strut for the front. You would have to swap the spring and bearing plate from your old strut, that's what the pan is for. That's the lower seat. In the rear, the spring is independent, so you just by the shock.
Personally I'm not into coil-overs. Every time you adjust them you're supposed to get an alignment, and I fix my own car to save money, not so I can drive an ass dragger with a fart can zip tied to the bottom.
You shouldn't have to replace much down there. You might end up with some space between the top shock mount and the frame, which is normally filled by a foam washer. You'll see it on there. Mine was all compressed, so I stuck a rubber washer in there and it worked out great.