The idea is that you own a percentage of the house so say 50% and the builder retains the other 50%. You then pay your mortgage and a small rent to the builder. it can either stay that way or, if you get a windfall or a good salary increase and can borrow more money then you can increase your mortgage and increase the percentage you own on the property.
I wouldn't go direct to a bank. Go direct to a good independent mortgage advisor as they will have the best deals available to you.
If you are Lothian, Borders, Fife I can recommend someone to you.
You would pay a mortgage on 50% and rent on the other 50%, and after 25 years you would own 50% outright and have no more mortgage to pay but would still need to pay the 50% rent. At that point (or perhaps sooner if you get a pay rise/promotion in the next 25 years) you might be able to take out a mortgages for another 25% and own a bit more.
The advantage is that once you retire, and have a smaller income, you won't have to pay the same level of rent as you would if you spent the whole time from now till retirement age renting.