Flash8
As clearly as it is part of our culture (unfortunately) to subconsciosly categorise each and every person we meet into the socio-economic categories you describe as lower-middle etc., it is also part and parcel that within these we have subgroups of people.
Interestingly though, much anthropological research has clearly shown that this categorisation does NOT depends upon the amount of money the person earns (slightly dependant upon our stoic British reluctance to talk about money) but rather on the way that person conducts and presents themself.
Unfortunately, along with the "lower-middle" you mentioned, also come "Chavs". Us British also have a tendency to support the underdog or the hardly-done-by too.. so derogatory or not, accept it exists and that there's nothing you can do about it!