Lender won't allow it. They'll want an interest in the whole house, they can't repossess half a house.
Only way to do it is a deed saying you have 50% equity, but then you would both need to be on the mortgage 50/50, they stop paying, you are liable.
I assume you don't live there? Will the other guy? You'll either both need to live there with a residential mortgage, or get a btl mortgage, but then neither of you can live there. Can't get a half and half
If it's not your main residence, then you need to calculate if you'll owe CGT.