You mean it is personally convenient or what precisely?

LHR is in a pretty stupid location, isn't it? The main flight paths go over London, which means it cannot operate during the night. Most modern major airports are to the North or South of the city they serve (rather than East or West) thereby avoiding this problem.

Additionally, LHR is in a much more hemmed-in location than many other airports, making expansion difficult, although this seems pretty inevitable given the size and density of the built-up area around London. Madrid, for example, is a totally different story: once you've driven about twenty minutes from the centre of the city, it's just miles and miles of open country.