Well, these are your options:
- You move into "his" house. It's only 1/2 his house. It may never be "your" house, you'll may have no say about what goes on and it could damage your relationship because of building resentment.
- You move out together (renting or whatever) and he'll be still paying the mortgage on the house he owns... so he'll be paying the mortgage on that while you pay the bills yourself (bearing in mind HE is still on the "property ladder", you are not). He could have a tenant move into his room in his house and that should help with his mortgage payments; or
- You buy a house, yourself, pay the bills yourself and he moves in with you and helps only with utilities (this is so he has no claim on the house... don't know how that works in the UK). You're on the property ladder, you have a chance to see how you handle living together, and then decide what comes next.
The house he owns will only be JUST the brothers if the brother can buy him out which you said neither can afford to do. Essentially the brother will be getting the benefit of the money your boyfriend pays for the mortgage and the entire house to himself. Your boyfriend will have equity in the house. Will be able to move back in if he needs (still his house and all) and also has a rent check coming (if he rents out).
I personally would do #3 otherwise you're putting your life on hold for "what if's". Yes it would be great to have your bf in a position to buy with you (though I wouldn't with "just" a bf), even better if you could wait until you're married, but there's a lot of waiting there and a lot of stuff depends on him and HIS life and choices. You could be waiting a long time.
Start looking for places now. Maybe move in with him until you find your own place or rent a place yourself (bearing in mind crossover of rent and mortgage can be $$$).
Congrats on a new path in your life!
p.s. So if you buying your own place makes you feel like you're not committed to the relationship, what is his owning his own place? If his brother wasn't involved would you ask him to sell it so you can buy a place together? Would you ask him to re-mortgage with both of you (and name on title) so you're included in it? Buying your own place is a really really good idea. This joint house with his brother is fine when you're a bachelor but eventually one of them will "get" the house to themselves and their family because 2 families living together never works for the long term.