The UK has a population of around 68 million. England, particularly the southeast and London, is densely populated; Scotland, Wales and Northern Ireland are much more sparsely settled.
Britain has been shaped by successive waves of immigration throughout its history — Romans, Saxons, Vikings, Normans — and more recently by post-war migration from the Caribbean, South Asia, East Africa, Hong Kong and Eastern Europe. About 18% of the population identifies as Black, Asian or from another ethnic minority background, a figure that is higher in London (around 40%). The country is genuinely multicultural, particularly in its cities, and this is reflected in its food, music and cultural life.
The four nations have distinct identities. The English tend toward understatement and a certain reserve, though this varies enormously by region. Scots are generally more direct. The Welsh retain a strong sense of national identity, with around 29% of the population speaking Welsh. Northern Ireland's communities have their own complex histories.
Class distinctions remain real, if harder to define than they once were. Accent is still a social marker, and the British relationship with the monarchy and tradition can be puzzling to outsiders. The UK is officially secular but culturally Protestant in England and Northern Ireland, with significant Catholic, Muslim, Hindu, Sikh and Jewish communities.
The British sense of humour leans toward irony, understatement and self-deprecation. If someone says your suggestion is "not entirely without merit," it's probably not good. If they say something is "quite good," it might be excellent.