I don't think £300 is a hugely unfair price for supply and fitting, as long as they are using good quality bits.
Decent pads are about £40 upwards, and a paid of decent coated discs £120 or so. So that leaves £140 for fitting.
Having said that in the past I have had good results mixing cheaper (But still coated) discs such as Comline or Juratek with good quality pads such as ATE, Jurid etc. If the TCR uses the same 340mm discs as my PP GTi then you can get a pair of coated discs for about £80
You may want to spend extra for better stuff if you track the car, but for road use they should be more than adequate.