Equal Treatment - Union or non-Membership
A prospective employee may be able to claim compensation from you in the employment tribunal if you offer him a position on terms that no reasonable employer wanting the job filled would offer and if the reason for such an unacceptable offer was union membership or non-membership.
Accordingly, if he does not accept the offer, or refuses to join or leave a union, this will be treated as a refusal to employ on your part and a claim can be made to an employment tribunal within three months.
If you are forced to discriminate in this way due to trade union pressure, you can join the trade union in any action against you. The employment tribunal can then decide how payment of any award of compensation is to be split between you and the trade union.