Is it illegal for my boss to use fire and rehire?

Yes, in the vast majority of cases, "fire and rehire" is heavily restricted and will result in an automatic unfair dismissal.

Under the Employment Rights Act, dismissing an employee to force them onto new, less favourable terms is deemed automatically unfair from Day One. As a fiercely independent, non-politically affiliated trade union, The Workers of England Union handles these disputes with absolute single-mindedness, fiercely defending our members' core terms and conditions without the conflicting workplace politics that often plague larger unions.