So if you don't like being taxed, why not go live in a city and stop driving? It's not compulsory.
I daresay the govt will find something else to tax if the savings on the health budget from more active travel aren't enough.
We are discussing principles here though of spreading the burden of tax evenly amongst the population.
The health benefits of cycling are irrelevant as that could be said for going to the gym - why not fund gyms instead of hospitals where the service is free at the point of delivery, it would be cheaper than funding a national road network exclusively for cyclists to use ?
If I sound unreasonable, that to me is the same as someone else demanding that people who live in a place which doesn't have a big Tesco/B&Q/etc on their doorstep to up sticks and move to somewhere which does (that sends a terrible message of big brother dictating don't you think ?)
For the record, I live in a place which has all of the big supermarkets, and all of the big DIY's within walking distance for the fit and able, and a town center (pop 120k) which I can walk to in 20 minutes. I don't work within walking distance though, and the buses don't run to suit my working patterns. I've always had to find my own way be it by car (and carshare), motorcycle or cycle commuting.