Take legal advice here.
Rush job before Christmas.
Missing insulation and possibly fire-checks.
Sadly, all too common in new-builds.
Sadly, NHBC are too busy with really bad stuff, whereas this is relatively minor, and they don't have the staff or the stomach or the guts to sort these things.
I saw one case almost exactly like this a few years ago...missing insulation was pumped in, and it filled the meter box too...
If the missing insulation was notified within the first 2 years then the builder should be made to rectify this, but you will probably find that subcontractors are blamed and the dispute will go on for a long time.
Sadly, there just isn't enough money to pay local authority BC departments to get the staff to check ongoing building works.
If I were in your situation, I would ask LA for a grant for the insulation, and have the meter boxes protected while the insulation is inserted.