The Saturdays have been very consistent, they haven't had a number 1 yet but there' a number of reasons for that. GA have only had 4... They are more respected than the Sugababes, because regardless of what the band achieved when the members had talent, they will always be remembered for the joke they have now become.
I agree with your point though, being British doesn't guarantee success in the UK. If anything US acts are just as popular if not more so.
I get what you're saying, but Amelle, Jade and Heidi are all undeniably talented.
I guess you could say that was bad luck. B'day wasn't well received here, it didn't flop by any means but B'day Bey was nothing compared to the heights she reached during the I Am... Sasha Fierce era. Look at its chart history, it was very consistent. http://www.chartstats.com/albuminfo.php?id=13797
Any idea how many international albums have spent 100 weeks on the chart?