Think my insurance told me that they will insure anything up to £5,000 without a vets certificate, anything over that will need one but they didn't say what stage. I'm with NFU.
Im with NFU and my last 2 horses cost £5k plus and both needed vetting certificates for the insurance. For my young horse I also had to give evidence that the Vet had done Xrays and that he was happy with the results (I think its automatic for anything over £10k).