Despite the stereotype, British people are not entirely uptight, and they are also not entirely drunken louts who only know how to have fun when inebriated (though there may be a few of those knocking around the United Kingdom). Indeed, if there’s one thing the British do very well, it’s throwing a fancy party that is still incredibly good fun.
One of the elements of this, perhaps counter-intuitively, is that there are an awful lot of mostly-unspoken rules about parties. The purpose of these is not to make people feel bad or like they have to work hard, but to ensure that everyone is included.
For example, if you have a dress code, the richest person won’t be much better-dressed than the poorest. If you have a seating plan at dinner, the most popular person can’t get more attention than the least. If you ask about dietary requirements or preferences ahead of time instead of just serving whatever comes to mind, no one is ever served food they don't want to eat. If you know how to measure a space, there are never more guests than the room can comfortably hold, and everyone is comfortable. If it's considered polite to arrive a little late, nobody ever feels bad because they got screwed by the traffic.
Of course, you have to be truly British, right down to the marrow of your bones, to pull it off the way the best hosts do, from the stateliest of stately homes to the humblest of Edwardian terrace houses. Let’s see if you have what it takes!