@Longanddrawnout
The only absolute guarantee is not to get married. You can marry and do all the wills and estate planning you want, but nothing will stop him from trying to get his mitts on your money and house should you die and it will cost a pretty penny for your DD (or her guardians) to fight him.
As far as divorce all bets are off the table. There would be nothing to stop him from trying to take you to the cleaners. My understanding is that pre-nups are 'considered' but not legally binding in UK divorce. Even here in my State in the US it's not completely unknown for an 'iron clad' pre-nup to be set aside under certain circumstances, and a financial 'imbalance' is one of them.
You really, really need to speak to a family law and estate planning solicitors. The former wrt divorce the latter wrt death. Only they can give you the real legal information you seek based on your own situation. Just be prepared for them to say there is no 100% guarantee.
If you do decide to go ahead with this, please name someone OTHER than him as guardian or trustee of your estate in case your DD is a minor or you wish to hold back a large inheritance until she reaches a certain age. And then be sure to update your estate plans periodically to keep up with new laws wrt spousal entitlement and DD's age.
As far as his 'inheritance', that's in the future and an unknown. His parents may need care homes. One may die and the other remarry and leave everything to a new spouse. That money is 'maybe'. Your money is 'real'. If I were your fiancé I'd be betting on the 'real' money.
Have you spoken to him about your intention of settling everything 'away' from him? If not, you need to. Because if nothing else he deserves to go into this marriage with his eyes open and the knowledge that he would be entitled to do the same if he chooses to in the event he does inherit a large sum of money.
TBH in your situation I would not marry, even if I lost the man.