You should replace discs and pads in axle sets, but one caliper can be replaced if the other side is in good order.
When you have to replace pads or discs, it's usual to examine the old components for any unusual wear patterns, and from that you can often identify problems with the caliper, and rectify it by refurbishing the caliper or carrier, or recommend replacement. You can also identify seized pistons during the process of winding back to fit the new pads.
If the OPs brakes are in this condition after 12 months, I would guess that this wasn't done, and a sticking caliper was not identified at the time. Either that, or there wasn't enough care taken to descale the carriers when fitting the new pads.
The problem with franchised dealers working on older cars is that they do most of their work on newer cars during the warranty period, when everything is shiny and clean. Fitting parts to nearly new brakes is easy, but a ten year old car with crusty brakes needs additional care, and a good independent will be used to that. Some dealers do not take that care, and I do speak from old experience.
Also, there is no way that spending a £1000 on a ten year old car's brakes is sensible. Find a good independent who should be able to do that work using aftermarket components for a third of that, less if it doesn't need a caliper.