This is the crux of the matter. Even with a gas boiler not achieving 100% efficiency, with a COP of 2 and leccy costing 4 times more than gas per kWh, utility bills are going to be double that of running a gas boiler. This will be a huge additional cost to the owner (think +£2k/yr)
If the Goverment want mass take up of ASHP then they should invest to reduce the price of electricity (and more importantly the spread of gas to leccy price per kWh). The £7,500 grant is just a gimmick.