I looked into this option in late November last year as it sounded like it could save a lot of hassle.
It did come with a few conditions attached which included:

a) part exchange value for your existing home is based on a "four week price", which is (as it sounds) a sufficiently attractive price that the house would definitely sell at it within four weeks. It is decided by getting the mean four week valuation from three local agents of your choosing.

b) The house you already own has to be "within the local area". I was unable to get an exact definition of how "local" the local area is, but if you are relocating from another part of the UK then you are likely to get turned away. I was surprised as at least one of the recent Taylor Wimpey developments in Colchester would consider part exchange anywhere in the UK. The reason is apparently that Cooks Shipyard is being sold by Taylor Wimpey East London which have a stricter part exchange policy than the Taylor Wimpey East Anglia franchise who manage some of the developments nearby.

c) Your current house can only be worth up to 70% of the value of the new one you are buying at Cooks Shipyard.

Obviously if anyone is disheartened by the above then it would be worth checking with Taylor Wimpey as they could have varied their conditions since November, but I would certainly have liked to known about the "only local area" policy before I had got attached to the idea of a part exchange.

Useful to know, thanks area_woman for that insight ;)