If a partner moved in with me they absolutely would help pay my mortgage. But I wouldn't lie that I didn't own the place. Just set standards of what I expect
My partner said that when he'd move in with me, he'd pay his share. His logic is that he's currently paying a landlord and he'd rather pay me. That way I get more financial room to loan money again (I own my apartment, but have a mortgage), and he'd pay less than current rent, allowing him to expand on a down-payment buffer. Ideally this way we could upgrade to a small house in time, suitability split, and I keep the apartment to rent out or I can sell it.
There is power in combined finances, but you need to take into account what you'd do on your own. That said, I would prefer to be in a situation where I could just let him move in for free, as life is expensive enough already.
But I also believe that it is essential to a good relationship that each carries their reasonable share. I grew up with my mom fully depending on my dad for finances as she was a stay at home mother. I loved she was always there for us, but when my parents grew apart, she really struggled and dealt with a lot of guilt because she couldn't provide for us as before. This is why I've always made sure I could make my own way. My apartment isn't the greatest but I'm thankful every day I can benefit from ownership.
All of Britain and maybe other european countries. Apartment or Condo here in North America.
Brits have a tendancy to simplify their descriptions. Like it is a "Flat" because the layout living space is typically single floor with no stairs. Or they will do the Hoovering of the flat. Hoover once being the go to brand of vaccum cleaner.