Unfortunately it sounds like there is nothing they can do. JAR FCL 3 states that the limits for astigmatism are -2 for initial and -3 for subsequent renewal medicals.
Although the CAA try to help as much as they can and will examine you to renewal standards if they think your vision won't deteriorate massively, one thing they can't do is to examine and pass you if you're already outside these limits, which you seem to be.