Thank you so much for your answers. I had a look at the documents and found them very interesting. Regardless of Degree Days, I specifically want to know the heating & cooling period in UK. In fact, I have to define the number of the days that the heating/cooling system should work in cold/hot period of the year. Do you have any reference for that?
Thanks for your answer. But how may I get it while I am not using EnergyPlus software?? To be precise, I am going to model some buildings in London (using another software), and I need to define how many days a year they need heating/cooling.