Us cyclists dont pay road tax so we should consider ourselves lucky.
Actually it's vehicle excise duty (VED), and it's for the permission to put a vehicle on the road. Cyclists have a legal right to use roads and don't require this permission. Also, if you work out 27 million vehicles at 165 quid or whatever, it doesn't exactly buy much when a mile of motorway costs several tens of millions. Roads are funded by a lot more money raised by general taxation.
ALSO, zero emission vehicles already attract a VED of 0 - and as bikes are zero emission vehicles....
Ride it or milk it!