From the Fast Stream website:
"Working outside London
A Fast Stream career does not have to mean London. Equally challenging opportunities are available in other parts of the country and offer a unique perspective which London-based Fast Streamers do not often see.
Even those in London are expected to be mobile and work wherever their departments need them. Remember, too, that in order to gain the all-important experience of operational delivery, you may well have to work in more than one location. Fast Streamers are increasingly being deployed to operational and corporate services work away from London headquarters."
So it does look as though it's highly likely you could be asked to go somewhere else at one stage. I'm not convinced it's compulsory, however - if your dept. needs you in London then they'll keep you there. Of the people I know on the Fast Stream its about 2/3:1/3 between those who have seen out the scheme in London only (with some short operational trips elsewhere, but no relocation), and those who have either asked for or been handed a post somewhere else [this is a relatively small sample so may not be representative, but seems to indicate some variation]. Suppose it just depends on individual departments and circumstances, but like Lee says, there is a requirement for a degree of flexibility on the part of the employee.