The simple answer to the question is that a summer duvet and a winter duvet are very different and made for very specific reasons and climates. Your summer duvet should be light, airy and fairly thin to give you enough covering in those warm summer evenings while allowing excess heat to flow through the duvet and into the atmosphere. On the other hand, a winter duvet should be thicker and able to retain more heat during those cold winter nights which can impact immensely upon the quality of your sleep and the duration of your sleep.
It is easy to forget that sleep is vital for our everyday existence and if you are too hot in the summer time or too cold in the winter time this will impact upon the quality of your sleep and your productivity at work and at home the following day. Many people in the UK suffer from serious sleep deprivation, much of which can be traced back to the style and type of duvet they are using at different times of the year.
So in reality you should have a thinner duvet in the summertime and a thicker duvet in the wintertime although the UK weather is not always that predictable!