All we can do is to suggest some things via our own knowledge and experiences from afar. But a broader observation is that your circumstances would not seem to be unique, and limited to you as an individual. rather, they would seem to apply to a rather broad class of firearms;. I would suspect that, in the UK, **** the USA, ML'ers are a very small group relative to modern firearms. Law makers are generally not that familiar with their unique features and capabilities, so laws get written that get applied to them somewhat unfairly.
I would really suggest that you be in touch with your national shooting sports associations to assist you in this matter. Chances are that there has been a similar circumstance before. I suspect that, while the laws get made by the elected officials, but it is staff that writes them, and staff that writes the actual regulations that are applied. It could be that you need the actual regulations modified, and a national organization is going to be in a far better position to effectuate that than you are as an individual.
Another route that is pursued here is to wage your effort in the court of public opinion. Newspapers, and other communications media. That's a much tougher and longer road, but if you want to go that route, it COULD get done. The risk there is that peoples' public opinions don't always follow their knowledge of the facts however,and it can be difficult to get attention by the media.
Best of luck to you however you choose to pursue it however, and please keep us posted.