If you haven't been, then go. If you have been, then go again. I cannot think of a single reason why the Natural History Museum in South Kensington should even be missed whether you're a hardened Londoner, or even a tourist in London for a short while.
Where to begin? It's free to get in. Admittedly, once in a while there is an exhibit you may have to pay for, but it rarely crosses the £10 threshold, and is always a treat.
The dinosaurs are fantastic, whether you're 6 or 60, the anamatronic velociraptors are eerily realistic (with a bit of imagination!) and the skeletons, millions of years old, take you on a journey to another time and place.
It took me quite a while to realise that the Natural History museum offered more than dinosaurs. For years I dragged my begrudging mother, weekend after weekend, to traipse around the looming monsters. But the museum offers so much more. The anthropology sections, the rooms devoted to creepy crawlies and critters (while not for the faint hearted) introduce you to a world underneath your feet.
Whether you spend a day, or merely a few hours, the Natural History Museum is not to be missed.