Hi :)
I have a problem with my muscles going into spasm and numbness and all that kind of thing but you can find ways to suit you for many things.
Your physio may be able to suggest some ways. Stretching exercises and those which can help posture can help, maybe something like yoga (something gentle like Hatha, Iyengar and Viniyoga, avoid Ashtanga) or pilates. How about swimming or some gentle water aerobics or stretching? Maybe after a while buy a bike and go for gentle bike rides somewhere nice, just built up the distance and such gradually.
Maybe join a gym and get an instructor to tailor a programme to what you can cope with, for example there are cardio machines you can use with your arms rather than your legs.
Just make sure you let any instructor know so they can maybe adjust exercises for you, you need to push yourself a bit to get the benefit but take it easy, don't rish or overdo it.
A good deep tissue massage therapist can help too and try putting the bottoms of your feet together and work on getting your knees down while keeping feet together which can open the passage through which the sciatic nerve passes.