The devil is always in the detail.
You say the rent is “slightly” below market level. I think that’s a “slightly” disingenuous slightly! £600 instead of £700 is a full 14% off. And that after increasing your rent by £50. So you were getting 21% off. Maybe not exactly if rental rates have changed.
So it’s a reasonable discount.
Presumably this has also been going on for some time? They bought it before you even met him - so he’s had this cheap rent for years, I expect - before he met you, and long enough for him to have moved you in. I’m not sure if you’re married, but I think you might be as you say the house is willed to “us”. You say that you’re now too old to buy... which does imply a reasonable passage of time. So I think that rental discount has had a chance to mount up.
You don’t say whether they have their own mortgage to pay from the rental income. You do say they’re wealthy, but that’s relative.
I’d be really interested to know if he paid more rent when you moved in? I’m imagining the AIBU... “we wanted to help our son out. He’s now moved his girlfriend in who works full time - WWBU to reduce the subsidising?”
Maybe they didn’t put up the rent after you decorated in a mean way because you’d increased the rental value. Maybe they thought, “hey - how come we are STILL subbing our adult son years after we bought his place, when they can anyway afford nice new carpets, are they taking us for a ride?”
None of that may be the case... but what I do think is true, is just because they have more money doesn’t mean they owe him subsidy. Rent at market rate of £700 means you don’t like in a super expensive area, so why exactly do two adults working full time still need that £100 from mummy and daddy?