I was able to get a certificate from my UK insurers, stipulating that it must be brief enough to download.
But it was a damn nuisance, requiring a number of phone calls.
I do urge you to keep your UK insurance but for the purpose of the Thai Pass get the Thai Covid-specific one. Although I paid 500bht for mine, i could have found cheaper versions. Some only accommodate youngsters below seventy-five, however.
I felt it best to avoid anything that may hinder, slow down or invalidate the Thai Pass application. I note that my Thai policy has been submitted and accepted by the powers-that-be.