This is interesting, as I'm also having trouble with my digital TV signal - with several stations listed (even after a retune) but then claimed to "not exist", with no signal. I also live in SW Lundun.
The problems with digital TV are 'atmospheric conditions' arising from a static high-pressure area over the UK. HD Freeview stations are usually the worst affected, and DAB can also suffer from 'bubbling mud'. On FM it manifests by getting 'foreign' stations, I've had ones from France, and even Italy (though that was a few years back). I don't think this is what causes the loss of presets on the car, but I'll be very pleased to be proven wrong!