There is no automatic entitlement to such a benefit, although if it could be shown that you were being treated less favourably than other workers on grounds, such as race, sex, or disability, you might well be able to lodge a claim of unlawful discrimination with an employment tribunal.
The provision of such schemes is a matter for the parties and is therefore a contractual matter. Any such entitlement should be clearly stated in your contract of employment as an express term.
Schemes approved by the Inland Revenue give exemption from tax on the value of the shares at the time of acquisition, but the realising of the asset value of shares, or on termination of employment is a complex matter and further advice should be obtained from the local tax office.
Just One More Click: Doggy drama about staying safe online at work.