Sickness, work and reasonable adjustments

Hi there, im new here and am looking for a bit of objective advice as to what i should do in my situation.

I work in a large retail chain as an assistant manager, i am salarised, so get the same wages no matter how many hours i do, but am supposed to do at least 39 (so says my contract).
I also get 3 months full sick pay and then 3 months at 50% pay (both at company discretion)
My job currently involves everything that needs done in a shop, doing schedules for staff, costing wages, security, ordering, filling shelves, bringing in deliveries, lots of manual handling.

Over the last 2 years i have undergone treatment for cancer, meaning i took probably around 6 weeks sick over that period, my last period sickness was around 6 months ago while recovering for a procedure.
My manager (and area manager more so) have been understanding about me requiring time off due to procedures and recovery, but i still spend most of my days off at appointments and scans and stuff as i dont want to "take the Michael" with time off.

Now in the last few months i have been developing symptoms of possible arthritis/ carpal tunnel, or something similar in both my hands, arms and shoulders, i fell over a few weeks ago pulling a muscle in my leg and have been hobbling around ever since, i didnt want to take time off work due to my previous time off.

I have visited a rhumatologist around 3 weeks ago, she checked me over, sent me for xrays, bloods and a ultrasound, im still awaiting ultrasound, but xrays indicated no problems with my hands or arms, but onset of degenerative disease in both my feet (im thinking now thats what made me fall over recently).

I told my manager i had bad hands and arms, and i had hurt my leg falling over, but his response was "what are you going to do while your here then as theres nothing else for you to do"
I responded that i needed to be on light duties until i get an answer from the rhumatologist as to what i have and what treatment is possible, so i could stick to mostly paperwork, price checks and organinising the staff within store rather than manual work, he wasnt happy at all, and continued giving me "to do" lists with loads of lifting and pulling of trolleys.

Ive been taking codeine and iboprofen for the pain, but every time i do manual work involving my fingers and hands, my whole knuckle area swells up a few hours later, and i am in excruciating pain for days later, im not getting proper sleep, im waking up all night with pain from my hands/shoulders or headaches, dry mouth from the codeine.

Im now at a point where i know i have something wrong with my hands and feet, but still awaiting official doagnosis, but i know i cant continue working in this job the way it is at the moment as the pain and impact on my life is unbearable.

Im wondering whats peoples advice is.
Would it be unreasonable for me to ask my area manager to allow me to do more of the admin side to our store, or even move me to another store that has more management support so that if i cant do certain things others can do those things instead.

At the moment the managers and asst managers are being used as the backbones of the stores as they can basically get you working loads of hours for no extra pay, meanwhile the company gives shops a bare bones wage budget for staff, with the expectation on managment to take up the slack.

I really dont want to go to them demanding unreasonable things, but my other options are go sick every time i cant work.

Any advice or recommendations would be great, thanks.