Well if you're income was £15,000 you could borrow £45,000. Although some companies will lend you more than that maybe up to 4 or 4.5 times your salary (£60,000 or £67,500).
If you have any unsecured debt (loans, Hire purchase, credit card etc) they will deduct that from what you can borrow so best to clear it.
Some companies will consider a guarentor arrangement, whereby someone else (eg your brother) guarentees to make the repayments if you stop.
You could look at getting a mortgage with your brother, but he would be equally liable and would have to stump up all of the cost if you were unable to. He would also have to agree if you wanted to sell the property/remortgage it to a loan in your sole name. And you would probably have solicitors costs to pay on those occasions. If he has a mortgage already that would be deducted from what you could borrow and he may not be able to help on that basis. It is a really big commitment for him to undertake (as I'm sure you know).
You may be better off trying to see if you could get more deposit together. Is there any relative who would be prepared to give/lend you the extra £40,000 or part of it?
A shared ownership scheme is usually offered by a local housing association (HA) and works by selling you a share the property, eg 40%. You then pay a rent for remaining share to the HA. When you sell you split the proceeds according to the arrangements you agree at the beginning. You can also have the option to buy extra shares in the future until you own your whole home.