This is all very tricky.
There are legal ways of dividing assets in a divorce.
There are legal ways of ring fencing assets in a Tenants in Common agreement.
There aren't many ways to protect yourself if you move into a house owned by another person when you aren't a tenant paying rent.
My advice is you look at other ways of covering yourself.
For a start, you should be able to ask enough rent to cover your mortgage and be in pocket (so you can save.)
You ought to be able to save out of your income (from work.)
BUT- rethink your idea of renting out your home.
One thing to consider is that the Labour government is going to make it far far harder to get tenants out with the end to no fault evictions. They could be there for up to 2 years if it went to court.
You will also be liable for capital gains tax on the house when you sell as it's been run as a business.
Unless your property is going to increase greatly in value, I'd suggest that- take financial advice- you consider selling your house and putting the equity into the house he's buying- in BOTH names.
That will increase in value more than yours and so will your equity.
He can buy you ought if he wants to stay there.
You can then be tenants in common and your money will be ringfenced.
I'd also query the 50-50 split on bills.
If he's the high earner, why are you paying the same amount?
I think it should be proportionate to your earnings otherwise you're always at a disadvantage of him earning and saving more than you.