Wow, was shocked reading this. I also swapped tyres out after a year and a half as I had a 2nd puncture on the Pirelli P-Zeros. I'm on 19" Alloys as I wanted the more cushioned ride. Size is 255/55 R19 W (111)
I went for Michellin Lattitude Tour HP's from a local garage in Leicester, costs was £135.00 fitted each, so £540.00 for the full set. they are noticeably quieter than the Pirelli and better performance in the wet too.
As for the price, my sister in law owns and runs a high end tyre garage in Milton Keynes, so i checked price with her before proceeding and was told it was a great price and she would not be able to match it. I also checked a well known tyre selling online retailer and there was only a small difference of around £5 per tyre which accounts for fitting and balancing.