Im not sure what its like for the more serious driving offences (like you mentioned the cut off point) but for the minor ones, the general rule is.....dont reply.
They dont have the manpower to chase up everycase and me and a few people i know have gotten away with it a couple of times simple because they couldnt be arsed to chase us for a reply.

If you do have to go to court. Say you got a son to visit and a business to run bla bla bla so they lower the fine and hopefully dont give you a week ban or something