No, it's an accurate model of reality, not a conspiracy theory.
In UK, the leadership of both Labour and Conservatives, are more cosmopolitan and urban (see the political theory about cosmopolitanism/traditionalism later in the answer).
Specific to UKIP quip, on the question of Brexit, David Cameron - leader of Conservative party - was firmly on the "REMAIN" side. So were most other Conservative leaders, e.g. his cabinet.
As per this BBC report, while among MPs of Conservative party, 185 backed LEAVE vs 138 REMAIN (over 50%), Only 6 of 24 Cabinet members were on LEAVE side.
Obviously, "ordinary people" is campaign rhethoric with no formal definition, but a rational approximation shows that what could be considered "ordinary" non-elite people did back UKIP's main point, exiting EU.
This article in the Atlantic did some breakdown of post-voting demographics. Their somewhat vague summary makes perfect sense:
The younger, unattached professionals of London favor European membership, while the poorer families living outside of major cities eye the continent with considerably more suspicion.
Politico's graph-heavy flash fest had by-degree breakdown.
Also predictably, people with degree were REMAIN by 70/30%, while those with High School only by 34/66%, while some college was ~50% split. Clearly, "ordinary people" can be assumed to be people with only HS education, as opposed to college.
... Only three of 35 areas where more than half of residents had a degree voted to leave the EU - South Bucks, West Devon, and Malvern Hills in the West Midlands.
Telegraph had what was probably best (closest to "ordinary people" model)
Class was a key dividing line in the EU referendum
Levels of education and class overlap strongly in the UK, and so the Brexit vote also matched up with areas with higher levels of people from the DE social class - meaning people in semi-skilled or unskilled labour, those in casual labour and pensioners.
This includes Blaenau Gwent in Wales, which has the highest working class population in Britain. Some 62 percent of voters here went for Leave.
Just three of the top fifty areas with the highest share of people from DE class backgrounds voted to Remain.
If you're genuinely interested (as opposed to just looking for an excuse to insult someone you disagree with politically), this article in RealClearPolitics explores the general idea, although on US side.
I won't quote the entire thing (too long, and much of it relevant) but just the intro:
There is an important but often overlooked divide that runs throughout modern western history (possibly other histories as well; I’m not familiar enough to say) – a divide between what we might call cultural cosmopolitanism and cultural traditionalism (the more loaded term is modernism v. anti-modernism). You can see it in studies of Stuart and early Hanoverian England, where discussions of so-called court/country disputes are central. You see it in various populist insurgencies throughout American history. It features prominently in the works of American cultural critics, especially the late Christopher Lasch
As well as the paragraph directly addressing the question, American-side:
Where this becomes relevant – indeed, I think this is crucial – is that the leadership of the Republican Party and the old conservative movement is, itself, culturally cosmopolitan. I doubt if many top Republican consultants interact with many Young Earth Creationists on a regular basis. Many quietly cheered the Supreme Court’s gay marriage decisions. Most of them live in blue megapolises, most come from middle-class families and attended elite institutions, and a great many of them roll their eyes at the various cultural excesses of “the base.” There is, in other words, a court/country divide among Republicans.
Specifically to address your "conspiracy theory" thing:
Conspiracy theory is defined on Wikipedia thusly:
A conspiracy theory is an explanation of an event or situation that invokes a conspiracy without warrant, generally one involving an illegal or harmful act carried out by government or other powerful actors. Conspiracy theories often produce hypotheses that contradict the prevailing understanding of history or simple facts. The term is a derogatory one.3
According to the political scientist Michael Barkun, conspiracy theories rely on the view that the universe is governed by design, and embody three principles: nothing happens by accident, nothing is as it seems, and everything is connected.1:3–4 Another common feature is that conspiracy theories evolve to incorporate whatever evidence exists against them, so that they become, as Barkun writes, a closed system that is unfalsifiable, and therefore "a matter of faith rather than proof"
Furthermore, Definition of "conspiracy" is:
a secret plan by a group to do something unlawful or harmful.
As such, none of this is a "conspiracy theory" since it doesn't postulate the existence of a secret, NOR a plan.
The mere fact that a specific class of people is in power, and has specific common interests, isn't secret. They also don't conspire in secret, they simply have same interests.