I think snail based boilies are a complete waste of time (in terms of trying to compete with natural snail consumption)... but if I was going to use a boiled bait with snail content I'd be using this:
https://essentialbaits.co.uk/products/black-snail
there was a company doing snail boilies It looks like they have stopped doing them, could be worth a phone call to double check
https://www.carpparticles.co.uk/product-category/boilies-hookbaits/