Whilst I do find London an amazing city in the Winter, my best time of the year to visit is in the Summer. This for me is the time when the city changes from being a quite serious but vibrant place to the best city in the world.
Things to Do
The sheer size and population of the city means that there is always a demand for things to do. With this on any give weekend or weekday for that matter you will find something to do. The city can handle major events like no other, take a weekend when there were the following events taking place: Champions League final at Wembley, (80,000 people), Rugby Final at Twickenham (85,000), Concert at the Emirates stadium (65,000) all taking place on the same day. There are so many events taking place in the Summer it is impossible to go to all of them, you have to sacrifice some over others and decide which you prefer. London Timeout magazine is a great website which lists the things to do every day in London and offers a great critic rating.
Places to See
The usual places to see are amazing all year-around but summertime is extra special, just be sure to make reservations where you can. Obviously the main attractions are the London Eye, Big Ben, London Bridge, Buckingham Palace and many others. If you are in London during August then the football season starts, this is a fantastic opportunity to experience the biggest sport in the UK. Choose from Arsenal, Chelsea, Tottenham Hotspur or West Ham to watch a Premier League game.
Whilst the tube is a great system that can get you around London extremely quickly, it can become slightly hot and busy during the summertime. With this I would recommend using different forms of transport maybe the Borris Bikes or Santander Cycles as they are now known Limousine would also be a good choice, to get around London by Limousine then I would fully recommend using as the best in market.
If London had good weather all year around then it would be easily the best city in the world to both live and visit.