I work for PwC as a consultant on the grad scheme. I can say you will often travel nationally (approx. 50%) of the time, but would be very lucky to travel internationally. This is because most assocaites like the idea of a project abroad. If you are more senior however the option is far easier to realise as senior staff are more in demand and have generlly more responsibilities at home (family etc) which mean less go for international opportunites.
No real difference between offices inside/outside of London and the likelihood of travel of the staff. It is a case of who you know, persistance and luck.
Hope that helps