I'm in a similar situation. Finding a good installer is tricky. I'm in a terraced property so similar there as well because there's very little space to install an air source pump. In an ideal world everyone would have 27mm piping but in my case I could get away with less and some better insulation. Sure it might not achieve the maximum efficiency possible but I'd hazard it's at least a better than our current boiler.

For now though I'm probably going to stick with the boiler with an OpenTherm stat to keep it running low. I'll try and bump up our insulation and hopefully things improve in a couple of years when the boiler gives up the ghost.