Can't go to Annual Leave from Sick Leave?

I was once told by my |NHS manager that I couldn't go straight from sick leave (several months following surgery) to annual leave, the day before I was due to start my annual leave. My annual leave had been booked before my illness. I thought using annual leave would reduce the leave that I was carrying over. My GP and surgeon all agreed I would be fit following my 2 weeks annual leave. So I simply told my manager that if she woudn't allow it I woud get another sick note. She phoned back to say I would be allowed. I have also since gone on a prebooked holiday during a further long period of sick leave. My job is very physical and going on any sort of holiday is not nearly so taxing! In fact allows me to build up my strength, swimming etc. Many illnesses can allow you to do some activities but may not allow you to do a stressful physical job. It is not as clear cut as some believe.