One can visit london at any time, depends on which climate you like. if you like cold then march is the best time to be there because London just comes out of winter . The seasons here is extremely varying. You can have heavy rain right now and it will be bright and sunny in one hour from now as if it is middle of summer.
If you want to enjoy UK go into the interior to see actual town & country life.
This is one of the places you will like the best if you
like beauty in flowers, sceneries. This place has maintained its natural beauty but at the same time has coped up with changing times in technology.
Beleive me it is not like US, this is a place where you will have combination of india and US. India in terms of people on roads, life just as in mumbai but at same time advancements in modern technology and cleanliness such as in US.
UK has been one of the best places I have been abroad. So enjoy as much as you want. If you would like to know more in details write a request to pramodsnayak@rediffmail.com