CGT due on sharesave scheme after using ISA allowance

Hi, I have looked through the other threads on this topic and there is not yet a clear answer to my question. So, I will approach this with an example and if you could explain how CGT would be applied to the example amounts I would appreciate it. I am in a 3 year sharesave plan with my employer that will see my total input amount to £18000 and it matures in the 2025/26 tax year. I anticipate the shares will be worth approximately double upon maturity, ie £36000. I will immediately transfer £20000 worth of shares into a stocks and shares ISA. I will then sell all the shares. This leaves £16000 which could be subject to CGT. My question is, how much of this £16000 is liable for tax? Can I deduct my £18000 acquisition cost and £3000 CGT allowance and therefore no CGT would be due? Please explain how CGT applies to the amount that will not go into an ISA.

Hi
If the £18,000 acquistion cost was for all of the shares you bought, then you would need to apply the relevant proportion to the shares you are selling. 
In this example if you paid £8000 for shares that you are selling for £16,000 your gain would be £8000. After the annual Capital Gains tax-free allowance of £3000 (assuming you have not sold or disposed of any other assets within the financial year), CGT would be due on £5000. 
As you are not disposing of, or selling the shares that you are placing into an ISA no CGT is due. If you choose to sell or dispose of them in the furture you may need to pay CGT then - depending their value at the time. 
More information can be found here: Capital Gains Tax: what you pay it on, rates and allowances