We recently had a load of letters in the post - one for each member of my family and countless others for people who have lived at this address over the last 20 odd years. The letters were from the local health authority and basically said ' if you want to remain registered with your GP then you MUST return this letter. If you don't return the letter then we will assume that you are no longer at this address and will de-register you .'
I'm guessing that it's something to do with the way the GPs or PCTs or whatever the jargon is, are paid..ie. if they have 50,000 patients listed but actually 20,000 have moved away but are still on the list, whoever foots the bill will only want to pay for the 30,000....or if they have to offer something (like vaccinations or smear tests) to a certain percentage of their 'customers' but some of the customers don't actually exist then the GP/PCT will not be able to reach the target.
Or I could be whittering on and talking rubbish cos it's late and I"m tired...but we did get the letters.