Take a look at HS283 here
for information on the main home and CGT.
First off, CGT was rebased to 31 March 1982. The value of your first property at that time will be your cost for CGT purposes.
Assuming that the first property will have been your main home for the entire period of ownership or for all but the last 18 months of ownership, you will have no CGT to pay on the gain. If you moved into your new property at the end of April, you would have until October 2016 to sell it completely tax free.
If, for example, you sold the first property 30 months after moving out, then only 12 months worth of the gain which is deemed to have accrued evenly since 31 March 1982 will be taxable. That's a small proportion. Assuming the property is jointly owned, you will each be entitled to exemption from CGT for the first £11,100 (current rate) of gains you each make in any one tax year.
Normally, you have one year from the date you buy a property to make it habitable, move in and have the first year treated as exempt from CGT as if you had moved in on day one. Sometimes, in extreme cases, HMRC will allow another 12 months.
You have two years to make an election for one of the properties to be treated as your main home to avoid the matter being determined by the facts.
I'd be inclined to move into the new home and try to sell the first one within 18 months.
I hope this helps but let me know if you have any further questions.