How much the builder pays you is not a VAT issue as such, it is a matter between you and them. If there is something contractually requiring them to meet your costs, then your costs are the £2,400.
But, if the builder had subcontracted the kitchen fitting to the company you have found before selling the house to you, they could have reclaimed the £400 VAT from HMRC. Therefore they might only be prepared to pay you the amount that it would have cost them to have arranged the work – i.e. £2,000.