For a start, it seems strange that a house 80% built has:
1) Not been released for sale and 2) Not been sold.
I think the sales advisor is making it up as they go along.
I would suggest that whichever plot you actually want, you call the area sales manager at the regional office and get a proper official price.
All sales staff have "negotiables" where they can give away "free extras" such as turf and or carpets and/or offer discounts around £5,000 to £10,000 on certain plots at certain times of the year!
My advice would be to not buy a new home and be aware that Taylor Wimpey have a terrible reputation for defects and customer care! Just because all the press is about Bovis at the moment doesn't mean the other housebuilders are much better in my opinion!
If you must, don't use the builder's preferred/suggested/recommended solicitor under any circumstances and have your house independently inspected and snagged BEFORE you legally complete and move in. Make sure it is a Freehold house too, not Leasehold.