it is dealt with under PAYE or income tax rather than capital gains tax, so 40% of the sum will go to HMRC.
Not the case if you are in the UK.
You will receive the money gross of any tax, and it's up to you to declare any capital gain in the year that exceeds £10,600. If you have exceeded this figure in the year by way of "capital gains", you are liable to pay tax at a maximum rate of 28%