Choosing the right time to visit Britain is not a straight-forward matter. A number of factors, including financial constraints, the weather, day length, and peak tourist times should be considered before you come to a decision.
Visiting Britain is the spring (late March to May) can be a real delight, as this is the time English gardens are at their best, bursting with blossom, bulbs and spring colour.
June to September is by far the most popular time to visit Britain. The weather is at its best (although still unpredictable) and provided you pre-book your accommodation, you should be able to find a place to stay amongst the crowds of tourists and holiday makers.
If you visit Britain during the Winter months (December to February) then not only will air fares be at their lowest you will also have relatively few other visitors to compete with, which means you can more easily find a table in a restaurant or a room at a Hotel. However, during the winter months the days here are shorter and the weather is cold, wet and often overcast, so if you are visiting the UK for sight-seeing, then this is particularly not a good time to come.
On balance the autumn time (late September, October and early November) offers much the same advantages as visiting in Winter (low air fares, cheaper accommodation and less congestion) but offers less disadvantages. Particularly late September and early October, when the weather is reasonably comfortable and more reliable, plus the days are still fairly long. A particular advantage is that the countryside in autumn can be delightful. During September we have spectacular sunsets and in October the leaves turn on the trees, spreading a rich blanket of colour across the countryside and along the roadsides.