I have bought a flat and decided to sell and not even live in it. It is a bit of a wreck and I have started the work which will increase the value a bit. Does this come under capital gains?
Then if you are not prepared to make it your "principle residence" then CGT will be payable. Bear in mind that the "Gain" is the difference between the purchase price (or valuation price if you did not buy it but it was, say, left to you in a will) and the selling price. Also bear in mind that expenses incurred in modernisation/developement are usually allowable to offset against profit and that the CGT allowance for 2013-14 is £10,900.
No, the rental you are living in would be treated as your Principle Private Residence. You have to take some basic steps to establish this with HMRC, which should include:
1) writing to them to tell them it is your PPR
2) ACTUALLY living in it. This means (at least) your mail should be going there.
However, as NJ pointed out, you can make nearly £11k of capital gain (and this would be the 20013/14 tax year now) in the tax year you sell it before any tax would actually be liable.
I suppose all this helpful advice on declaring your principal residence is presuming you're not an MP in which case this would be highly unacceptable 'house flipping'