If it's of any help in your battle with your installers the relevant part of the building regulations for external pipework is section 4.26 of approved document L: https://assets.publishing.service.gov.uk/government/uploads/...

The table in there assumes pretty high quality insulation and the neoprene-looking stuff doesn't meet that standard so would have to be thicker. Note that the way the performance of pipe insulation varies non-linearly with the pipe and insulation radius so it's a bit unintuitive.

How do I know? I had the same problem with my installers.