I thought that the Middletons were upper-middle-class?
(Sorry for delay - have been off-grid for a couple of days.)
'The British class system is unspeakably complex, being not only multi-stratified horizontally but also segmented vertically. It's important to grasp (your profile gives no clue to your location, so forgive me if you're a Brit and know this) that it's not a matter of how rich you are, but your education, outlook, manners and lifestyle. Also, like your investments, your class status can go down as well as up - traditionally, if you adopt a chavvy occupation and way of life, you will drift downwards and your more genteel relatives will cease to invite you round.
No, the Middletons are not upper-middle-class, although huge efforts have been made by some commentators to insist that they are, with ill-informed waffle about 'ties to the aristocracy' and 'they entertained royalty'. The Leeds Middleton family were solid Yorkshire middle-middle for generations, but Papa Middleton and his father both made a living as commercial pilots (very lower-middle) so in spite of going to public school he was a rung below his grandparents' level even before he married Mama Middleton, who of of course is pure lower-middle (builder's daughter from Southall, state schooling, air hostess) and they became very rich running a business selling party supplies, which is very definitely not an upper-middle or even a middle-middle-class thing to do, but nevertheless provided them with the money to launch their children at high society. Some journalists have adopted the term 'Upper Middleton Classes' for this innovative social phenomenon.