Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
This will not be an HMRC approved scheme so you will be subject to Income tax (IT), not Capital Gains Tax (CGT), on any gain made between the disposal price and the vesting price. This will be at your marginal tax rate. It is common for such grants to be sold on day of vesting thus precluding any capital gain.
I do hope that you find my reply of assistance.