Sick leave is sick leave. Your employer cannot tell you to use your holiday allowance. It's explicitly against the law https://www.gov.uk/taking-sick-leave
Note too that if you were sick during holiday leave, you can take that as sick leave and continue to have your holiday allowance.
It sounds like you work for a crap company that's breaking the law to exploit you.
Sick leave is sick leave. Your employer cannot tell you to use your holiday allowance. It's explicitly against the law https://www.gov.uk/taking-sick-leave
Note too that if you were sick during holiday leave, you can take that as sick leave and continue to have your holiday allowance.
It sounds like you work for a crap company that's breaking the law to exploit you.