As far as I can tell, there's no official guidance on the distance you can travel. That said, you should try to stay as local as you can. The Gov guidelines say that "if you do leave home for a permitted reason, you should always stay in your local area - unless it is necessary to go further, for example, to go to work. Staying in your local area means stay in the village, town, or part of the city where you live." Travelling to an MOT station that's relatively nearby should be fine, just don't go too far.