Over the years I have found MyTyres very competitive on price. I avoid using their fitting stations as my local ATS will fit them quite cheaply, and I know they will do the job without damaging the alloy rims. When online tyre firms use fitting stations, I suspect the price they pay them is no doubt peanuts, and I would not want possibly having the apprentice loose on my wheels. I learnt the hard way with an alloy costing a lot more than an Omega alloy rim, by which time it is too late when the damage has been done.
Blackcircles give their fitters £14.
I am reluctant to order from one place and fit at another because:
a) its always more expensive
b) its a nightmare if there is a claim on the tyre - I've had a few go faulty over recent years.