Backdated payment

Hi, 

I received a work for capability assessment from UC after being on limited capability for work for a few years  in 2020. And I didn’t receive an appointment due to Covid and I put in many complaints about this through my online journal and they kept telling me to wait and I’ll receive an appointment. Fast forward to June 2021 after getting frustrated of waiting to long I formally requested a change in health to force them to give me an appointment and I finally got an appointment and got put into the LCWRA. But I’ve only been backdated from the June 2021 date. And UC have said that’s the date from when the claim started. Is that right because I was under the impression that because I’ve been waiting for an appointment that never came since 2020 that’s the date I should get backdated from because UC started the process of reviewing my LCW by sending me a WCA assessment form which I sent back on time. And I was told to just wait for the appointment when I enquired about not receiving appointment. Am I right that I should get backdated from the 2020 date? And is there law surrounding this issue?