It is not necessary to start with the preparations for the trip to London too far in advance. The most important thing is to bring an adapter for the plug, since there the plugs are not the same as in the rest of Europe. As for the time of year, spring is recommended, since it is when the best temperature is. You have all the information in: «What to keep in mind when planning your trip to London?«.
You have all the information in: "What to keep in mind when planning your trip to London?".