You are joining a company not a fleet so just take whatever you are offered, get in the front door and you can move around later till your heart is content - the date you joined the company means everything - not the date you join a particular fleet.
Not only is this good advice for your benefit / career, but also in "definition" terms under BA's agreements. In your first 5 years, BA can shift your fleet/base at their whim. So sign a 744 (LH) contract, and on day 1 they'll show you an Airbus (SH) CBT and say, "arrr, yes, things have changed"
[and it has happened in recent times]