You might refer to HS283 for information on the main residence and CGT. In particular, Example 9 is of relevance to you.
There isn't much you can do I'm afraid. If you put the property into joint names with your elderly parent for example to divide the gain, you would lose half the tax free gain you are entitled to as a result of the property having been your main home.
As the property was your main home, the gain for that period will be exempt from Capital Gains Tax as will the gain for the last 18 months of ownership. In addition, as the property has been let as well as having been your main home at some point, you will be entitled to a further deduction from the gain called letting relief which will be the lesser of:
2 the sum of the gain for the period you lived in the property and the gain for the last 18 months of ownership and,
3 the gain for that part of the letting period not covered by the last 18 months of ownership.
If there is any gain left after the deduction of main residence relief, the last 18 months relief and letting relief, the first £11,100 of what is let will be tax free due to the annual CGT exemption.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your income in the tax year you dispose of the property. Take a look here to see how to work out how much of the gain will be charged at 18%, 28% or a combination of the two.
I hope this helps but let me know if you have any further questions.