This question then becomes about your principal private residence (PPR) and the relief from capital gains tax liability that is possible.
By the way, I fully support what ubasses had to say in his first response to you.
The house you have been living in and will move out of gets PPR relief up to the time you move out plus 18 months. It used to be longer but is reduced. So if you rent it out in October 2015, any increase in capital value from April 2017 and the date you sell it would trigger a tax liability. You can offset any liability against your personal annual CGT allowance.
The flat you move into becomes your new PPR as of Oct15.
There's a decent explanation of it with more detail of the caveats here
http://taxaid.org.uk/guides/information/other-taxes-you-may-need-to-know-about/capital-gains-tax/selling-our-main-private-residence