Contrary to popular belief, it does not rain every day in the United-Kingdom. However there is always the possibility of rain no matter what season you're traveling during, so it is always advisable to bring some type of waterproof clothing.
Winter is generally cold and wet so it is wise to bring clothes that will keep you warm as the temperature drops to below freezing during the evening and is normally just above freezing during the day. Snow is not uncommon but do not be disappointed if you don't see any.
During the summer you will normally be comfortable in pants and a T-shirt although there are days when shorts would be more appropriate. Bring some warm clothes as the temperature does fall in the evening.
The summer months are from the beginning of June to the end of August.
Spring and autumn are very unpredictable. The temperature will vary from warm to cold so research the weather before traveling.