It would appear that each Surgery maintains details of its own catchment area and that these are closely guarded secrets, which makes life difficult for anyone moving into the area.

When they were looking for a use for Louise House, I thought it would be an ideal place for a GP's surgery and mapped all the local surgeries to show how poorly served Forest Hill was. You can see the map here. However, NHS Choices have improved their website in the intervening years and you can now sinply enter your postcode to find the nearest practices which are supposedly accepting new patients.

Unfortunately you still have to ring round and ask whether you fall into their catchment area.

If you have problems getting registered, then (depending on where you live) contact either Lewisham, Southwark or even Lambeth PALS as Ghis and BarCar have already mentioned.