When this sort of thing happens, simply explain to them that:
Legally, you have just as much right to use the road as they do. (unless its a motorway)
You dont pay road tax
You're harder to trace should you be seen cycling illegally (RLJing usually, but sometimes speeding) due to the lack of number plates
You dont have to fork out for insurance (this ties in with my previous point, which would otherwise effect it if caught)
Petrol price rises dont affect you
You get fit, they get fat
The government is on our side, the government is at war against them
Your arse hurts and its THEIR fault, not your seat