Lancashire is one I know of with only C's and no D's, E's etc. Devon (IIRC) goes down to F's. As I said, unlike A and B roads, C, D, E, F, G, U, Q, X etc. are purely for local administration purposes without any set statutory definition, generally they end or change designation or number at administrative boundaries - a bit like public path numbers on definitive maps for that matter.
I agree I oversimplified what LA's may know about individual routes. This varies greatly but it is generally fair to say that unless there has been cause to investigate this (eg. for a definitive map order) they tend not to have too much info. Highway records have always been concerned primarily with maintenance liability rather than levels of public rights and this was a major reason for the creation of definitive maps of public rights in the first place. If an authority does believe they know the level of rights over an unrecorded route (and it is of a type which should be recorded on the definitive map) then they are neglecting their statutory duty to record it. This is actually now a very pressing issue. As it currently stands, on 1st January 2026 - a mere 10 1/2 years from now - all footpaths and bridleways which existed before 1949 (ie. when the definitive maps were created) but which are not recorded on the definitive maps will cease to be public highways. Non-vehicular highways need to be properly recorded ASAP.
RUPPs were recorded as part of the first definitive maps. The idea was to record very minor vehicular highways which were better considered as part of the ROW network than the road network. The eventual definition in the 1949 Act "a highway, other than a public path [footpath or bridleway*], used by the public mainly for the purposes for which footpaths or bridleways are so used" with its apparent focus on use rather than rights caused instant confusion. The recording of RUPPs was very inconsistent and very incomplete. Many areas simply didn't bother recording any vehicular roads or omitted many because they weren't much used by walkers or equestrians, some others recorded footpaths or bridleways which ran along private access roads as RUPPs
LA's were required to reclassify all RUPPs by the Countryside Act 1968 (47 years ago). The act stated that the draft revision must be done within 3 years of the Act
Rights of way have always been low on some LA's list of priorities and with many LAs not anywhere near completing this process in the intervening 38 years, parliament redesignated the remainder en masse as Restricted Byways (a newly created category) in 2006. I'm intrigued to know how you feel that motorists were advantaged by this process.
*Public Path was defined elsewhere in the act as the collective term for footpaths and bridleways.
Salsa Vaya by Alyrpal, on Flickr
. Sometimes if you delve the local authority says they have "limited liability for maintenance", but still don't state what the right is. There are also "G" and "Q" roads, (see 1.4 
Many were reclassified as Public Bridleways rather than Byways. Sadly I think the reclassification gave more "right" to motor vehicles, but that's a personal view.