There may be some interplay with Impostor Syndrome, but IMO what is being discussed here is a different thing, for which there may be a more specific name... but if so I'm unaware of it.
Impostor syndrome is pretty much universal (insomuch as it crosses class/economy boundaries) whereas what has been mentioned here is a sort of anti-entitlement very specific to people who grew up poor. People in this situation are often afraid to ask for what they are worth (to the point where they will sabotage their careers to some degree as noted in Gustomaximus post), are generally afraid of authority even when there is no reason to be or the authority isn't actually an authority over them, etc.
I've definitely seen and experienced this phenomenon myself speaking as someone who grew up fairly poor -- single-mom American poor (which, as all things are relative, is quite different than seriously-might-starve poor... at least for now).
It sounds a little like learned helplessness ... having spent some time in the UK I have noticed this unquestioning deference to "ones betters" and I wonder how it comes about. How is this trait acquired?
I got it from my father, who rose from poor to wealthy through education. He taught me that education was all that mattered. When I achieved a prestigious Silicon Valley position after dropping out of college, I considered myself a fake who would be discovered at any moment for a long time. All my colleagues had compsci degrees. Thing is, I eventually discovered that I could hold my own. What my father taught me is only one way to achieve success, as I later realized. But that he taught it as dogma crippled me for a long time.
It is acquired socially. The UK has a formalised class system dating back to the "divine right of kings". The "ruling elite" are rather literal as the membership of the upper house (House of Lords) is limited to hereditary peerage.
92 out of 813 seats are hereditary peers, the remainder are life peers [Edit: forgot the Bishops] appointed (technically) by the monarch on the advice of the government.
There's really no such thing as a class 'system', let alone anything one could point to as being 'formal'. There is a bunch of in/out group signalling with fairly extreme geographic variations. Mobility across social boundaries can be achieved by understanding and adapting to the signalling although crucially (and getting back on topic) this requires the belief that you are entitled to do so.
I'm down on the notion of class in general personally. I think it's at best an incredibly lazy intellectual model of complex social phenomena. Basically I think there's no such thing [0], but I do very much enjoy listening to the things people say while they're explaining why I'm wrong about this.
[0] Problematic for this theory is that people certainly believe there is and this has operational effects.
https://counseling.caltech.edu/general/InfoandResources/Impo... https://en.wikipedia.org/wiki/Impostor_syndrome