Depends how much over £15K? Most of the usual insurers will insure up to about £30K. I think KBIS may do a sports horse cover or contact Lycetts insurance brokers who deal with bloodstock/sports horse cover. They will have certain providers in the market (QBE may be one of them) who only deal via brokers. I think there is also BRIT insurance but I am not sure if you can go direct with them or need to go through an insuranxce broker.
i would get quotes from all companies. i would also suggest you get quotes for a 5k horse. LOU will stack your premium sky high. a good policy for 5k should still give you vets cover and public liability. the difference in premiums would be a bankable amount.(for future replacement etc) and if a mare LOU might not pay out if still breedable from,eq the same for a stallion. and generally you would only get the value insured for if horse breaks a leg or doenst come through colic. if the horse is paddock sound then you would only get 60% of insured value anyway. whatever way you are damned if you do and damned if you dont! enjoy your lovely horse, insurance is a fall back mainly for vets fees in my case. dont know if high value horses can have high value vet cover. lets hope horse doenst break!