All of the hourly weather data that comes with DesignBuilder is supplied by DOE from:
www.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm
But with the year changed to 2002 as described in Add new hourly weather data.
Most of these weather files are composite files from multiple years. Each month has been selected to be representative of that month for the period of record. Representative of the normal patterns for that month not the average. The selection of months is usually based on a weighting of temperature, humidity, wind, and solar. There are some variations among the weighting, most are similar to the TMY2 done in the US in the 1990s:
http://rredc.nrel.gov/solar/pubs/tmy2/
You can find the details of the file format in the AuxiliaryPrograms.pdf EnergyPlus document.
You can download future climate change weather data for the UK in EnergyPlus format from:
http://centres.exeter.ac.uk/cee/prometheus/downloads.html