Legally, an employer has to pay Statutory Sick Pay after the first 3 days of any sickness absence, but this is only about �63 p.wk. If this is your situation, you might have felt it better to treat the absence as hoIiday and get full pay for it, but this should not have been a decision solely for your employer.
If your employer has a contractual sick pay scheme which pays more than SSP then the employer is in breach of this term of the employment contract by telling you to take the absence as holiday. You can complain and maybe take them to an Employment Tribunal.