Any small house builder would love to have a buyer signed up at an early stage, especially in this market.
You would need to look in estate agents windows in the area you are interested in.
All the larger housebuilders have schemes where you can reserve a new home "off-plan". Some call it "early-bird" some "upfield reservations"
With the larger housebuilders, a house not started may not command the discounts you are looking for.
They will be more interested in selling stock homes and these get the discounts.
There is also the question of timing. see
www.brand-newhomes.co.uk/when_to_buy_a_brand_new_home.htmSee also
www.brand-newhomes.co.uk/new_home_news.htm#Buyingoff-planRegarding finding developments there are lots of property finder websites. You could try the Zoopla one advertised on the above page.
Trawling a council planning website would be the easiest way in my opinion.
Finally, be aware that you need a specifc date of when the home will be started and more importantly when it will be finished.
In the current market you may end up paying more for the home by reserving early than you would if you wait.