Honestly, this all just seems to stem from the ASHP breaking down, and possibly being undersized in the first place.
If it as having to run on resistive the whole time, then yea, it would cost a blooming fortune.
Building regs on heat retention have also tightened by 20% since 2020, so newer builds should require less to heat them now.
What I don't get though, is that if it was a newbuild, would it not be covered by NHBC's warranty? Which should include the heating system, unless it is already under another warranty.