How do employers choose who is made redundant?

Your employer should use a fair and objective way of selecting people to make redundant. This means that it should be based on some evidence, rather than your employer just deciding who they want to give notice to.

Source: Money Advice Service

Redundancy selections and notice periods @ Direct.Gov
Excerpt: One method your employer could use is to consider which group or section of the workforce the redundancies will be selected from. This is called the ‘selection pool’.
After identifying the selection pool, the employer should apply selection criteria to it, to narrow down the employees. Your employer should, as far as possible, use objective selection criteria that can be applied equally and fairly across the workforce. Examples of selection criteria could be: disciplinary records, experience, capability, and relevant skills and competence.