3 are s**t no matter what backup provider they use.
the fact it switches from 3 to o2/orange when it looses its signal, which cuts calls and takes 20 seconds a time to do, is whats annoying! as well as 3 customers having less priority to the backup provider meaning multiple attempts to dial out or network busy errors. very very annoying. (only talking from experience)
from personal experience O2 is the best, which is why iv gone back to them. iv tried them all but tmobile and my ex rated them very s**t! she lived in manchester city centre too.
orange is ok, voda is good ok too from experience.
depends where u are. in london for exmple, they will all be good because there is loads of coverage. where i am there is a lot of land and fields, so a good SP is important. O2!