Thanks for all the responses everyone - I won't name and shame either the Insurance company or the Garage publicly, on the off chance something comes back to bite me, but given there are relatively few insurers in the UK who claim to have specialist Tesla policies, and most of them are rebrands of the same company, I'll let you guys try and figure it out.
The latest is that they called me today to close down my complaint after I sent them the full email trail of communication between me and the garage. I promptly refused to let them close the complaint, as there has been no resolution, reminded them that they are MY insurance company, supposed to act on MY behalf, and not protecting the garage.
I have now requested that the insurance company a) escalate the complaint b) remove the car from the garage by Friday this week c) move the car to a more local garage d) return the car to me, fully repaired and inspected by Tesla by Friday next week. I've also suggested that, since they chose this garage over one that would have been closer to my home, (most likely due to kickbacks in a contractual arrangement), and against my wishes, that I will hold not only the garage but the Insurance company liable for any battery damage that has occurred.
I also spoke to Tesla today again, to see what their view was on lead times for the parking sensor and wiring loom that are supposedly delivery date tbd, and they said there were no supply issues for parts in the UK that they were aware of, most things were being supplied from stock at the moment.
Needless to say, the car will be inspected with a fine tooth comb as and when it is returned to me, and any inkling of a blemish/imperfection/anything out of place, it will be returned to the insurance company to rectify.