Leave this with me while I draft my answer.
As you are no longer an employee of the company, you cannot claim entrepreneurs' relief which would have limited any CGT charge to 10%. In any event, you would have needed to own at least 5% of the voting shares unless the shares were acquired through an Enterprise Management Incentive Scheme.
There are few ways to defer a gain or wipe one out completely if the asset being sold is not a business asset I'm afraid.
You can defer a capital gain and get income tax relief by investing in an Enterprise Investment Scheme. You can read about EIS here and here. They are a high risk investment vehicle and really only suitable who can afford to lose all their investment.
You can also invest in a Seed Enterprise Investment Scheme, get 50% income tax relief and wipe out a capital gain forever. Read about SEIS here and here. This scheme has been extended to 5 April 2017. Again, this is a high risk investment vehicle, hence the generous tax reliefs.
CAPITAL GAINS TAX
There are two rates of CGT, 18% and 28%. The rate or combination of rates you will pay will be dependent on the sum total of your income and taxable capital gain in the tax year of disposal of the property. Assuming you sell the shares in the 2014/15 tax year, one of the following will apply:1 If your income in 2014/15 including the taxable gain is £41,865 or less, then all the taxable gain will be taxed at 18%.2 If your income in 2014/15 excluding the taxable gain is more than £41,865, then all the taxable gain will be taxed at 28%.3 If your income in 2014,/15 excluding the taxable gain is less than £41,865 but more than £41,865 when you include the taxable gain then part of it will be taxed at 18% and part at 28%.I hope this helps but let me know if you have any further questions.