Advice needed re adjustments at work

I have to attend a meeting with my employers regarding my sickness absence. 

Several of my sickness days were directly due to depression and anxiety and were reported by me as such. However I'm finding it more difficult to cope with working full time - I get very tired, sometimes feel overloaded and get anxious about certain things. This often makes me "run down"  and more liable to get viruses and infections. 

I have decided that I need to reduce my hours significantly, as I feel much better and more able to cope if I have a couple of days holiday during a week, so I want to reduce my hours to part time. 

My GP assessed me using the AQ test a couple of months ago and offered me a referral for an "official"  diagnosis, but was of the opinion that I didn't really need it because I was able to work most of the time. I'm going to consult with him again next week to ask for a referral after this development. 

What I need to know is - what are the options if my firm can't offer me reduced hours?