I went to MCM in London about 10 years ago, possibly more. It was pretty much just a nerdy flea market as Jinx described. Although they seem to have upped their game in recent years and tend to have things like discussion panels, cosplay contest etc so it's more in-line with some of the bigger US cons now.

Smaller cons are just pretty much nerdy flea markets. Bigger cons are an insane crush of people. Then there's something like gamescom in Cologne which is just insane levels of magnitude of people.