What they "should " do, and what they actually do, is sometimes far apart. I had a horrible experience with Nationwide, they said that because I was going to have a child NOTHING I earned could be taken into account, because I could lose my job and would have to pay for nursery ,so they then said my Childcare fees would be counted AGAINST me, like negative income. It was insane. I showed them my Employment Contract quoting three months' notice vs my Nursery Ts and Cs stating 1 month's notice, pointing out that if I couldn't find another job inside the 3 months (I work in finance, highly employable) then I would simply quit the nursery with plenty of time to spare. And, as we could easily afford our mortgage on my husband's salary alone, my income was only ever going to be used for overpayments etc.
Nationwide refused to accept this - which was utterly absurd - and told me I couldnt have a mortgage. I argued, and argued, and complained, but they dragged their feet until I was at of my house falling through.
I should have gone to the Ombudsmen or someone and complained, Nationwide behaved appallingly. I got a written letter of apology from a senior manager in the end but nothing else. I almost lost the house I desperately wanted. Ended up with HSBC who were wonderful, saw my financials with OH and said "sure, you can have a mortgage" - agreement in principle took just a few minutes.
I would not touch Nationwide EVER again, it was the worst anti women discrimination I ever encountered and I have been bitter about it for 5 years.