Following with interest. Obviously horses in fields are a slightly different environment that poultry in sheds - but it might be an interesting addition to the battle.
Of course you can't! Parasitic wasps are not that discriminating and the last thing any of us should want to do is bring in a predator which is US based to UK soil. We have - hopefully - learned the lessons of myxomatosis.
But both the bio fly control you have cited and Aprt fly control are UK based. But they are also only aimed at controlling the type of flies around poultry sheds. This does not make them suitable for the massively wide range of flies the UK Horse population faces. As I said in another thread - to be effective you have to understand and target the type of fly you have an issue with. But if you are stabled close to mass poultry units this will absolutely be relevant.