If your going to apply, BALPA members only please!!!!!! (BALPA are going through proceedings for recognition at Jet2)
Article 1 of chapter 38 of the 1990 Employment Act makes that illegal.
The act which can be read in full
here specifically states in the very first article of said chapter:
Access to employment
1
Refusal of employment on grounds related to union membership .(1)
It is unlawful to refuse a person employment— .
(a)
because he is, or is not, a member of a trade union, or .
(b)
because he is unwilling to accept a requirement— .
(i)
to take steps to become or cease to be, or to remain or not to become, a member of a trade union, or .
(ii)
to make payments or suffer deductions in the event of his not being a member of a trade union. .
(2)
A person who is thus unlawfully refused employment has a right of complaint to an industrial tribunal. .
(3)
Where an advertisement is published which indicates, or might reasonably be understood as indicating— .
(a)
that employment to which the advertisement relates is open only to a person who is, or is not, a member of a trade union, or .
(b)
that any such requirement as is mentioned in subsection (1)(b) will be imposed in relation to employment to which the advertisement relates, .
a person who does not satisfy that condition or, as the case may be, is unwilling to accept that requirement, and who seeks and is refused employment to which the advertisement relates, shall be conclusively presumed to have been refused employment for that reason.