A350-1000 isn't available until, when, 2015? Add in programme delays (not unreasonable) and BA's policy of not being on the 'bleeding edge', and you will not see an A350-1000 in the fleet until 2018/19 at the earliest...
The 77W purchase allows BA to (a) park 744s and (b) get cheap capacity due to 787 delays.
And come 2015, when there will be maybe 10 or more 77Ws in the fleet, BA can make an informed call on more 77Ws, A350-1000s, or 777NGs (whatever they make look like).
Today's purchase seems the only obvious call!