I don't think it's either S.S or SFT cuz there's nothing in those perfs in common with any Brits' perfs.
I agree with you on it must be from her third album [Britney] and the live tour [Dream within a Dream]
I still try to look for the performance from Brits, I'll post it here if I find it.
I think I've found the Brits' perf. Thanks to DareDB's hint.
Oh, well, what do you know, I was thinking of the Britney performance after all. I was thinking BoA was the one dancing with screen like a mirror, but I guess I confused the two of them in that video. Again, it has been a very long time since I've seen that video. Thank you a lot for clearing it all up.