what is the slang word for rich but uneducated people? especially those who live in rural areas and who like to show off?

The more genteel way of describing such people is to employ the French words 'nouveau riche'. At least that is what the educated class in Britain might call them. But if you want a slang term, one might be 'spiv', particularly applied as in 'spiv landlord'. But that has largely gone out of fashion. It is difficult to think of a universal term of any kind which might be used today. To denigrate such people would be seen as politically incorrect, almost tantamount to a form of racism. 'Hillbilly' is rarely used in Britain perhaps because such a social class never really existed. People who have expensive accoutrements, such as cars, but not much substance in education, might be called 'a bit flash'.

A big city person might call them a "Beverly hillbilly". If they aren't really rich but act like they are I hear "hillbilly rich" used - example is a person with a $40K car and $20K home.

Country folk might say they are "too big for their britches" or "high cotton".

The journalist H.L. Mencken coined booboisie (a combination of boob and bourgeoisie) to characterized the uneducated and uncultured people he believed ran the US in the 1920s. While it is not exactly a well-known word today, I believe the word is evocative enough to make its meaning known in context with little help.

Parvenu. It means a person from usually a low social position who has recently or suddenly become wealthy, powerful, or successful but who is not accepted by other wealthy, powerful, and successful people.