Westminster Abbey has more history crammed into a relatively small space than any other place I can think of.
The Abbey has been the site of countless coronations and weddings, and doubles as a graveyard for the great and the mighty (the two don't necessarily overlap). The bodies of Isaac Newton and Charles Darwin and the original Queen Elizabeth I and Mary Queen of Scots and Edward I (Edward Longshanks of Braveheart fame) and a host of other luminaries lie there.
Both Elizabeth and Mary have their carved likenesses on top of their coffins, which lay above ground. This allows you to see what they looked like when alive. (I sorta wish this custom had continued.)
There are also a number of people who weren't truly great, but merely prominent religious and political figures from eras past. Churchill and Disraeli are commemorated there, but so are a host of other politicians you've never heard of, including many who were undoubtedly the Harry Reid's and Nancy Pelosi's of their day.
But the fact that Charles Dickens and Rudyard Kipling and Alfred Lord Tennyson and George Frederic Handel and David Livingstone are buried there leaves you with an overwhelming feeling of being in the presence of greatness, even if their bones have long since turned to dust.
There are also memorials to people not buried there, like Jane Austen and William Shakespeare and Anthony Trollope. It's almost as if England tried to compress all of its glory into one cathedral.
(It's hard not to wonder which recent figures will eventually be immortalized there. Will John Lennon and Paul McCartney make it? Christopher Hitchens? Andrew Lloyd Webber?)
Many of the bodies buried there lie right beneath the floor, and it's hard to avoid walking on their tombs. Darwin's grave, for instance, is stepped on by thousands of people every day. It feels a little disrespectful, but it's pretty much unavoidable. (At least no one pisses on his grave.)
The only grave one is not allowed to step on is that of the Unknown Warrior, an anonymous soldier from WWI whose body lies beneath it, surrounded by poppies. While the rest of the markers leave you feeling awed, this one leaves you sad, at the thought of this young man and all the others like him who gave their lives so nobly, and so wastefully, in their youth.
It's worth taking the official tour. The guide will point out things that you'd never notice on your own. For instance, that Sir Robert Peel organized the nation's police force, and this is why British policemen are called "Bobbies." Or that Lewis Carroll's memorial in Poet's Corner is circular so as to symbolize the hole into which Alice fell in Through the Looking Glass.
Our guide was an older, somewhat fastidious fellow who always asked if we had any questions, but tended to answer them somewhat impatiently. At one point he pointed out that William the Conqueror, otherwise known as William I, who was crowned in 1066, was the last king to be given a title with a descriptive name (like Edward the Confessor, or Ethelred the Unready). After that, they were merely given numbers (like Richard III). I tried to trip him up by asking when Richard the Lionheart reigned (I thought it was after 1066, and sure enough, it turned out to be in the late twelfth century). Our tour guide huffed that that was more of a nickname, not an official title.
But at least he spoke with an English accent. Given today's London, we should have counted ourselves lucky that he didn't speak in an Indian, or Russian, accent.