Hi volavoka,
As you did not sell the shares immediately upon vesting, you may be liabile to pay Capital Gains Tax on the disposal of shares.
The gains will be based on the disposal value minus the acquisition cost, which would be the value of the share when your employer awarded the share to you.
Please have a look at the guidance here:
ERSM180020 - CGT Interface
In relation to capital gains.
Thank you.