The point being it is £20k more for a home install after the fact. It is about £4k for them to be installed by the builder. And further to that the point being the regs are the issue, pushing for regs that are half assed is the problem. Not the builders directly. Also we have worked with 2 or 3 smaller developers that have already gone ground source so it isn't completely impossible but regs need to be better written and more stringent.
To that end a coupe of other developers are looking at alternatives because air source are not suitable for the level of clientel and they are likely to swallow cost and indeed increase the sales price to cover.
And yes better regs are needed for insulation/cavity increase, triple glazing, UFH, solar and PV etc. But the starting point should be at least considering the minimal footprint homes have with gardens and such ground source heat pumps rather than air source.
Your post reads like you're complaining that home builders are doing the minimum to comply with regs, which is to be expected unless you are buying a somewhat custom/bespoke home. Where are you getting £4k install costs by a home builder? Sure maybe for an ASHP but the issue with GSHP is the land space you get with most new builds is small so you have drilling and boring costs (since theres not enough space for trenching), plus the management of that project pipeline to do. This is the reality for most new builds, we're not talking about one off new builds in large plots etc as they are far from the norm to the majority.