Any seized bolts could be cut away if all the parts are being replaced anyway. The only 'difficult' if damaged thread would be the pinch bolt which holds the shock into the steering knuckle bit it could be sorted out.
Is the quote of £320 for one side only? If your not familiar with all the above parts, to change a spring means removing all of thse parts anyway so they can't charge you any extra labour for having to remove the shock absorber or top mount for example.
So at a guess I would reckon £150-170 would cover it at any reasonable garage if only for one side