Late spring / early autumn are the best for general touristing. The weather is less dark&dour than in winter, attractions are less crowded than in Summer, and hotel/accom costs are lower.
If your main goal when going is mostly to party though then summer is the way to go. Drinking Pimms in a park in summertime is one of the more enjoyable aspects of life in London. Plus there are always lots of tourists to hit on in the summer months, which is nice.
If your main goal when going is mostly to party though then summer is the way to go. Drinking Pimms in a park in summertime is one of the more enjoyable aspects of life in London. Plus there are always lots of tourists to hit on in the summer months, which is nice.