The decision for the DEC to launch an appeal for any country in the world is, in a large part, about unmet need. Large scale disasters can affect anyone, but for countries that have suitable infrastructure and resources for disaster response, international humanitarian aid is usually not called for. In many cases the country affect may not even call for international humanitarian assistance if they have a framework in place.
While people in countries like the UK may be struggling after an emergency, there is funding and a framework to help them get back on their feet and return to their every day lives. This is not to say the DEC does not want to see help provided to those affected, but there are systems in place to ensure they get help without a DEC appeal. Also, as most of our member agencies won't be working within the affected regions, there are no direct programmes for DEC funds to go to.
One of our members, the British Red Cross, are usually involved in assisting British people who are caught up in the most difficult situations. If you go to their website at http://www.redcross.org.uk/Where-we-work/In-the-UK you can see where they are providing help and support.