[Red Herring]

Exhausts that do not conform to regulations, ie: they do not have the BS/E markings and especially if they have "Not for road use" or "For racing purpose only" are techniclly illegal and they can be dealt with in several ways. You can be reported for summons in the old fashioned way, this means the copper has to write a report and submit a file (not likely unless there are other jobs as well), they can issue a fixed penalty (£30 no points) OR they can give you a rectification ticket. Technically when they issue you with a rectification they are reporting you for the offence, however provided you fix the fault/put it right within 14 days then they will not submit the file. They cannot give you a fixed penalty and a rectification. It's also worth bearing in mind that the rectification does not mean you can ride around for 13 days without getting done again....you're back in play as soon as you ride away.
Altering the exhaust to try and make it look legal, ie: polishing out the markings or adding your own, is a crime rather than a road traffic offence, however they do need to prove you did it, which without an admission on your part might be hard, unless you were daft enough to buy the plate on your credit card....

As for number plates anything smaller than a barn door is illegal, however guidelines are that if it can be read at 20.5m then it should be advice only.... however they are guidelines and you're being an @rse expect them to get bent.