Why do you want to leave your property to your partner and not your children? You could will it to him as it is but he would be liable for IHT, if you marry he would dodge this liability but if he were to inherit everything, he could then leave everything to his kids and nothing to yours?
If it's long term planning for IHT then really as I said above marriage works well. However it opens other concerns. A prenup isn't unrealistic, but a private client solicitor and a suitably experienced IFA will give you your options and the associated costs. If marriage is off the cards then these are reviewable provisions.
I'm an IFA not a lawyer, but when we talk to clients solicitors we are told: No, but can be taken into account. A prenup has more chance of being accepted in certain circumstances..... one of them being, it exists!