I think I've got this. When you pay the Fee your exhaust emissions are automatically cleaned up, filtered etc.I had to pay my fee when I went to the bike show in Tobacco Dock a couple of years ago on my disgusting filthy 2002 motorcycle. I made sure I got my money's worth by razzing it around parliament square a few time at full throttle
I F'ed off before the rozzers woke up and pulled my over though.
Ulez? Utter bollex and nonsense.