Do you have any concerns about the operation of the brakes, or is it just the state of the discs that is concerning you? It's not unusual to get areas on the edge of the pads where the discs rust, especially as the discs age. it is not a symptom of a failing caliper. and so long as the brakes are operating normally, not usually something to worry about until it is mentioned on the MOT. To be honest, they don't look that bad to me - I've seen a lot worse.
As there are no obvious signs of seizing, I doubt there is anything wrong with the calipers, but a set of discs and pads might be in order soon if they are getting scabby - parts cost £30-£40. Unless KwikFit have actually stripped the brakes, I can't see how they can diagnose whether the problem is a faulty caliper, sticky caliper slider pins, or just sticking pads.
Brake fluid should really be completely changed every 3 years or so, it's the best thing you can do to preserve the life of your calipers.
The creaking of the parking brake is very common, and not something to worry about if the brake is otherwise working normally.
Not sure what the prices are like on the Autodoc, but you can buy rear calipers for less than £45 each from UK sellers on ebay, if you do need them. I've fitted a number of these cheap pattern calipers over the years, two on the front of the Jazz, with no problems.
I absolutely agree with Jocko above, try to find a good independent garage - I know it's not easy at the moment, but try to ask around for recommendations. I would avoid the larger chains such as KwikFit, Halfords, etc., they are set up to part you from your money.