ways to increase the amount to borrow

Hiya, my partner's basic wage is about £40k and this year with over-time he has had about £53k but if you put that into the Halifax calculator it says we can borrow about £210,000 but if you say his basic is 53 you can borrow much more.
We need to move in about 2 years and by then he might be on 53k but not sure yet. His boss will also retire in a few years and my partner would then be a business partner. How does that change what amount we can borrow?


Thanks