If you sell your old house within 1 year of moving out of it (on the basis that it was your principal private residence while you did live there) then you won't pay any cgt on a gain on disposal.
If it is sold more than one year after you vacate it, then part of the gain will be subject to CGT at 20%. The amount which is subject to tax is time apportioned but you are deemed to have had it as your PPR for one year after you move out. So, if you own the house for four years and have lived in it for two (and rented or left it vacant for two before selling) then 25% of the gain will be subject to CGT.