Extremely unlikely that the pads are worn out given the milage you have covered.
It just needs a brake service / clean contact areas and then brake lub applied to the ears of pads and the mating surfaces of the callipers, you know, the lub the factory never installs

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Just be a little guarded here, if the dealer gets their hands on that job, they don’t use the opportunity to whack you to replace the pads and discs, which is going to be a very expensive job ( £900.00 ish ).
Just a thought, If you intended to remove the original pads in the same locations, then why the need to push back the pistons ?.
Unless you are fitting new pads of course !.
Are you 100% sure the squealing is coming from the rears and not the fronts by any chance ?.
Of course, working on brakes should only be tackled if you have previous experience of this type of work and the necessary tools and skill sets required here

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