You might want to call the ppl who do your horse and rider insurance, or even your car insurance. They will definitely ask for the value of the trailer before they will be able to give you a quote.
had a quick look and SEIB says £120, seems cheap but then its only cover for damage or if its stolen.
Do many people have a trailer and not insure them?
Stoneways do and so do E&L, just hads quotes off both of those on the internet to insure my IW. But couldn't be bothered with the faff to save £3 so just renewed with datatag who it's currently covered by as it's got a datatag chip