That might hold as an argument if it were not for the fact that even at trade prices the Suprimas are not that good a choice.
Apart from the very low powered units if you take a middling 60 kBtu/hr (c. 18kW) unit with the flue and the VAT you are talking about the same money as a system boiler from Vaillant. The latter has the pump, filling loop and all the sealed system components included. (That alone would be pushing £100 + fitting labour).