I suggested to consider Rituximab.
I got rituximab after I couldn't get an approval for IVIG. But my story is complicated, it’s not just for SFN...
My doctor could prescribed both of these treatments off label, and then it was either getting the approval of the insurance company, or paying for it by myself.

I don't know how is it in UK, here it seems that Rituximab is less expensive than IVIG. the price of a single infusion is roughly the same, but according to my neuro it might take 10 IVIG infusions or 2 Rituximab infusions over the first 6 months to estimate whether it helps.
So if your doctor is ready to prescribe IVIG and Rituximab, but you can't get the approval of the insurance company you might consider paying for rituximab.