Likely a difference in terminology.

"Upper middle class" in the UK comprises the top 5% or so of the population. They tend to be senior professionals or business owners, are likely to be privately educated, will probably speak with a "received pronunciation" (rather than regional) accent, and have significant asset wealth.

"Upper class" is reserved for landed gentry, nobility, etc. They're people who can live off long-standing inherited wealth and don't need jobs or even education (though many still do have them, of course).