I'm insured with Locktons (last four years) BUT........they will only cover you for a trackday if it's a PCGB one.
This may well have changed over the years, but I haven't had any updates from them.
When I went to Spa for the first time, I took out cover from a specialist trackday cover company ( sorry dont remember the name). It was £86 for covering a 986 valued at £5k.
I use OpenTrack for my days and as a comparison, Goodwood full day with them is £275 to £325 depending on time of year. Compare this against the PCGB ones at £495 for the day.