hi there Rolac,
yes, in theory, BA can move you to whatever fleet or base that they want to within your first 2 years of employment with them. but after that time BA has to respect your choice of fleet and/or base.
HOWEVER
i have never heard of this happening within the last 9/10 years. if you do join eurofleet at heathrow as permanent cabin crew (which is what they are recruiting for at the moment), it's 99.9% guaranteed that you will remain there for that 2 year period.
the only time that you really have to worry about being moved to another fleet/base is if you join the company as temporary cabin crew.