Unfair redundancy
There are several reasons for dismissing someone or selecting them for redundancy that are deemed “automatically unfair” for which a tribunal does not have to consider whether the employer has acted “reasonably” — and the claim automatically succeeds.
These automatically unfair reasons are generally characterised as asserting statutory rights or acting as an employee representative — for example, selecting someone because of their trade union activities, or because they have made a protected disclosure.
Targeting someone for redundancy is not in itself automatically unfair (unless the reason you were targeted is one of those protected categories). But you could have a claim of “ordinary” unfair dismissal on the basis that redundancy was not the real reason, but a manufactured one.
If your employer cannot show that it had a genuine reason to dismiss you — one of those listed in section 98 of the Employment Rights Act 1996 — again the question of reasonableness will not come into it because they will not have met the first test of a fair dismissal which is a fair reason.