You should refer to the HMRC helpsheet HS283
for further information on the main residence and Capital Gains Tax.
As you know, for disposals of residential property after 5 April 2014, the last 36 months of ownership exemption will be reduced to an exemption for the last 18 months of ownership.
The gain for the period the property was your main home will be exempt from CGT as will the gain for the last 18 months of ownership. So, if you sell it before the 18 month period expires, you will pay no CGT. If you sell it 6 months after the 18 month period expires, then 6 months worth of the gain will be taxable subject to the annual CGT exemption which is currently £10,900 per individual.
If you let the property for 24 months, then you would qualify for letting relief which would reduce any part of the gain not covered by the main residence exemption and the last 18 months of ownership relief by the lesser of:
2 the sum of the main residence gain and the gain for the last 18 months of ownership of the property and
3 that part of the letting period gain not covered by the last 18 months of ownership.
Letting relief will apply to each part owner so is potentially worth £80,000. Clearly, the figures are critical in this and every case is different.
You could make an election for the property you are thinking of letting to be treated as your main residence for CGT purposes but not if you let it. It cannot be your main residence by definition if it is let as it is not available to be so used. You have two years from the date of acquisition of the second property to make such an election. Where a property is jointly owned all part owners must sign an election. Of course, such an election would potentially expose part or all of any gain you may make in the future on your new property to CGT.
I hope this helps but let me know if you have any further questions.