jde is right till 2004-2005 BA would tell you what base you were going to fly from only mid-course!!! So...not old timer at all, AMS-LHR!!!!
I speak for myself and my colleagues we fought a lot for people like you to know where they were going to work before they applied/started working for BA.
Having said that they can give you a contract let's say based at LHR EF but send you to WW for a secondment or any other combination possible. They have the "right" to send you to another fleet during the first 2 years if requirements dictate so.