Nearly all housebuilders expect buyers to exchange contracts with 28 days. This means paying 10% deposit, which is non-refundable if you back out for whatever reason afterwards.
The exchange 10% deposit would be made up of your own savings and/or the exchange deposit from your existing home which you would need to sell first if you haven't got savings.
The home isn't built yet and you are buying off-plan. By the sound of it you haven't even seen Bellway's brochure yet. The price could and often does go up as the home is constructed so buying off plan means you benefit before even moving in. But be ware you could be locked into buying at a HIGHER price is the market falls which is could do with all this Brexit going on.
You may have an opportunity to book an "early bird" where you get first refusal on a particular plot when it is released for sale. This might give you time to at least exchange on your current house.
You may even be able to get a housebuilder to part-exchange on your existing home.
My advice would be NOT to buy one of the first homes on a development. Even better, not to buy a new home at all until a New Homes Ombudsman is set up by government.
