Furlough while on sick leave

Hi 
I'm looking for some advice and hope someone can help. 

I have been off work since January 2019 due to surgery on both knees, I am now disabled due to complications and chronic pain. 
I received SSP for 12 months and this then stopped so I claimed ESA as my employer doesn't pay sick pay. The ESA payments will stop in July.
During my time off I have been paid holiday pay on a monthly basis by my employer. The amount is exactly the same each month as I get paid pro rata. In April the holiday pay amount dropped by exactly 80% so I presumed I had been furloughed. I was not told this by my employer and have recieved no correspondence with regards to furlough and have not signed anything. Is this right, should I have been told that I have been furloughed? 
I have had no contact from my employer since I have been off. Ì have completed my obligations as an employee throughtout and sent my fit for work notes and updates on my progress but I have never had a reply. 
My employer has finally contacted me this week and said that due to the length of my absence they need to assess if I am fit to return to work which may involve an occupational health assessment. I understand this but I feel thoroughly let down by my employer and feel like I have had no support at all during the time I have been off. 
I am not fit enough to fulfil the duties of my role so I know that I will not pass any health assessments. Would it be best if I resign? 
I'm a bit confused and also suffering depression and severe anxiety which my employer also knows as I have also kept them up to date with my mental health struggles. 
If anybody could please give me any advice with regards to my employers obligations to me with regards to furlough I would greatly appreciate it. And also what my next move should be in my response to my employers contact this week. 
Many thanks