OP, I pulled out our old offer from May, which was going to just be the other half on the mortgage. On the offer paperwork it says:
KievLoverTwo and anyone else aged 17 or over (other than you - they mean my partner) who will occupy the property at completion, must sign our consent to mortgage deed before completion. The consent must be witnessed by a solicitor/licensed conveyancer in all cases where the conveyancer acting for you considers this appropriate.
Then I found the form online, which says:
Occupier(s) Consent to Mortgage I/We will be in occupation of the Property at the time the mortgage is made. I/We agree with Nationwide that any right of occupation or share or interest in the Property which I/we may have now or later are postponed and subject to the rights of Nationwide as first mortgagee
Ergo: Nationwide's rights to the house come before your own. I see it as similar to being a 'permitted occupier' on a lease instead of on a joint lease; if they want to throw you out, they can.
https://www.nationwide.co.uk/-/assets/nationwidecouk/documents/about/information-for-lawyers/forms-and-downloads/m55-mortgage-deed-for-england-and-wales.pdf?rev=7f25cb12702349989df5b4fc8011ca42
I'm not a lawyer and the cynic in me wonders if your other half could get you removed from the property if things go wrong; after all, you won't be on the deeds or the mortgage.
Whatever happens, please protect yourself. I'm not even sure being married or in a civil partnership overrides the lender's ability to take the house off you if you sign such a mortgage declaration.
You could perhaps ask in Legal on MN.
If you to find out one way or another, I'd really like to know too, so will you come back to this thread, please?