Bit late to this thread, but Dodgepot and Chris are quite right; Xenomania are the key. Sticking with the same producers makes their albums so much better than most pop LPs. This is especially pertinant with the Sugababes comparison, in that they've released some great singles with Cameron McVey, Richard X, Xenomania, etc, but their albums never sound so cohesive as GA's because they mix styles via differing producers. The same happens in American RnB and hip hop, where a dominant theme and style can make an album (see Aaliyah with Timbaland, Kelis with the Neptunes, Snoop with Dre, etc).
As for the concept of "needing to write their own stuff", I'm sure Elvis, Frank Sinatra, The Ronettes, et al would beg to differ.