I was in a similar position and discovered that I could pay for an MRI at the local university, it wasn’t as much as I thought it would be (£300 ish). I too had a prolapsed disc and I’ve been doing physio led pilates for about 18 months that has helped enormously. Be careful about who you go to for pilates – there are pilates classes… and there are pilates classes. I would only ever go to a physio led one in your position.