I have a different answer.
Unless you elected for the holiday cottage to be treated as your main home within two years of buying it, you won't escape CGT for the last three years of ownership. Any gain on a disposal now will be fully taxable.
The first £10,900 of gains made by an individual in a tax year are tax free. The rate or combination of CGT rates (18% and 28%) that you pay will be dependent on your income in the tax year gains are made.
If the home that you have owned for twelve years has been your main one for the entire period of ownership, any gain will be fully exempt from CGT. See HS283
A married couple who are living together can only have one main home between them. In certain circumstances, it is possible to split the main residence exemption on a time apportionment basis depending on the circumstances though it would be unusual to make such a claim for a holiday home. In any event, why would you want to sacrifice part of a full exemption for the property that you have owned for 12 years?
If you sold the home you have owned for 12 years (or within three years of moving out) and lived in the cottage for a year or so that might allow you to claim CGT exemption for the last three years of ownership. However, if the tax office consider that your motive for moving in was purely to avoid tax they may seek to disallow any such claim. Occupation of a property you claim to be your main residence is more about about quality of occupation as opposed to length of occupation. Quality of occupation basically means moving your life to a new home.
There really isn't much you can do to reduce your exposure to CGT apart from selling when you are not paying higher rate tax, ensuring properties are in joint names to get multiple CGT exemptions or putting property in one spouse's name if that spouse would pay less CGT on the whole gain than the two of you would separately.
I hope this helps but let me know if you need clarification.