No you can't do that (offset improvement costs) unless you are running it as a business.
But what do can benefit from (and given the timescales with propose this looks like a real winner for you) is the knowledge that HMRC allows the final three years of ownership of a property that is not your 'principal private residence' (PPR) to be exempt from any capital gain. The way to do this therefore is to declare with HMRC that your new home is your PPR as soon as you buy it, and make damn sure you dispose of the old home within the three year period. None of this matters, of course if there was no CG on the old house during the period when it is not your PPR and outside the '3 year grace' window, and you would still get your annual CGT allowance in any event, even if there was a CGT liability.
All in all, you should feel confident in avoiding having any CGT liability.
Some stuff on PPR and Private Residence Relief here.
http://www.direct.gov...ntalIncome/DG_4020890