Trapped nerve, signed off work, sick pay is a joke

I work 7:30 - 4:30 5 days a week as a callout electrician.

Got a "nasty trapped nerve" (Physio's words) around 3 weeks ago, meaning anything AT ALL involving my left arm is complete agony.

I also had a newborn son that same week so have been off on paternity for 2 weeks.

After 2 GP and 2 hospital visits all they have done is dope me up on a mix of cocodamol, naproxen, omeprazole, diazepam, and tramadol. These did next to nothing apart from make me drowsy. I paid for a private physio £126 for 2 sessions who alleviated some of the pain with a massage and some exercises for me to do at home. Currently waiting for an MRI scan which apparently could take months. The GP also recommended I don't work.

But the pain is still here and this week I had to return to work as paternity leave is a joke. £180 odd quid a week when my mortgage is £1300. I'm seriously struggling at work and I can't really ask them to lighten the work load more than they have, the job by nature is inherently physical. Even driving is agony. My fingers and elbow are completely numb.

Sick pay is around £90 quid a week which is even worse than paternity pay.

What the hell am I supposed to do? I've worked since I was 16 and paid all my tax and national insurance yet feel completely shafted. Add a newborn baby into the mix and I'm seriously at breaking point stress wise.

TLDR: Cannot afford to live on the peanuts that is sick pay, work is agony due to a trapped nerve that no one can fix. Is there any help available at all?