OfCOM OfTarget again
If this applies to fixed broadband and landline telephone services only, then it's not about ISPs - it's about Openreach. ISPs (including BT Retail) order installations from Openreach, via Openreach's work management system, and Openreach frequently fail to show up at the appointed time/day. You can't hold ISPs responsible or penalise them for Openreach's failures.
It's depressing that policy makers, and especially OfCOM, seem to be endlessly unable to take into account how the infrastructure and workflow actually operates when making stupid STBDS policies.