Seeing that you are in the UK, you certainly have your own health system obstacles to overcome.
I have been in and out of a power chair and spent a long time in a power chair. If you can move, definitely move. BUT if you need a rest, definitely rest. Movement helps with your muscle memory. Stretching will also help. Just try to do it daily to make it part of your lifestyle.
Every August for the past several years I have pain which presents in my left leg and lower back. I use a heating pad and apply it at the area of most severe pain. Heat causes the neuro transmitters to stop sending signals which eliminates my pain. This does not help everyone and your legs will feel weaker. The pain is severe and I have learned to accept my leg weakness in lieu of the pain.
I do not know if it will help, but the Royal MS Society in the UK might be able to help you. I realize that you are not diagnosed, but they should still be able to assist you.
I have had MS since I was at least 14. I was finally diagnosed when I was 19. MS is chronic, but it is not fatal. You can still enjoy a career and over 2/3 rd's of MSers in the US nowadays continue with employment. Thank you for sharing and please feel free to ask for help and seek support herein.