Three years ago I paid two fellas £600 to do a straight boiler swap for me. It took them about 6 hours or so and they got the boiler for me for less than I could buy it plus they registered it with Vaillant and Gas safe etc. £100 an hour for two blokes is bloody cheap in my book. My boiler was in the loft and for comparison, British Gas wanted a flat fee of £600 on top of the install price for either a tower or scaffolding just to flip a trim ring on the boiler flue that was coming out of my flank wall. My blokes shinned up a ladder to fit it, took no time at all.
As others have said, if you're not happy with the price, go elsewhere.
As others have said, if you're not happy with the price, go elsewhere.