Green’s Dictionary of Slang

an aristocratic or upper-class individual; the inference is derog.

1983		J. Byrell (con. 1959) Up the Cross 73: She [i.e. a boat] was as sleek and toffee-nosed as the posho flyer she’d been named after.
1991		Independent 9 Jan. 28/8: Socially, football has been followed by the lower orders, while golf and cricket has attracted poshos.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 67: [T]he poshos clustered around Lady Cynthia scattered like fat fairy penguins.
2001		Observer Mag. 19 Aug. 3: The government is petrified that Liz’s 50 years of service will pass uncelebrated, except by poshoes and wonks.
2014		Eve. Standard (London) 18 Sept. 44/4: Johanna Thomas Corr loves a posho.