If Sarah is to be believed then the girls always intended to split at the end of the tour. What really bothers me about this scenario is that all the girls, except Nadine, therefore deliberately, repeatedly and consistently lied to the fans during the entire reunion by saying they would not split up or at the very most, had not decided anything. I would rather believe this was a last minute decision to split which Nadine could not, or did not get the chance to try to fight. The alternative, is to believe that we fans were lied to, manipulated and deceived throughout the reunion by the other four girls!
I think it is obvious it was planned for months, Cheryl said in her book before the reunion they would have a children in needs single, a greatest hits album and a tour, then that is probably the end for GA. They will have all been aware of that, so they all lied, not just four of them. They maybe felt it was better to have a fun reunion together then split, rather than putting a downer on it by announcing the split at the start, plus if they had they would probably be accused of just trying to make more money out of fans.
The only thing that might have changed near the end is them announcing an official split, but I think it is clear they all knew they would be going their separate ways again after the tour.