The word from some BALPA reps (and from the ex-head of pilot recruitment a few years) ago is this:
BA only ever recruit into positions for which there isn't a suitable internal candidate.
A suitable internal candidate is someone who:
1) meets the hours requirements (unfrozen ATPL for LH)
2) is un-type-frozen (when you join you are frozen on type for 5 years)
3) actually wants to move into the new postion
In practice this means that BA only ever recruit DEPs for longhaul if there aren't enough guys on SH that meet requirements 1 and 2 above (there's always enough to meet 3
)
During the last recruitment drive there were many on SH who wanted to move to LH but not enough had served their 5 year type freeze - hence DEPs going LH last time.
This time however things are different. Due to the recession and stagnation in the seniority list (due EU age regs which came in in 2006) SH guys have not been moving to LH at the end of their SH type freeze. There is a long queue of internal BA SH pilots who have unfrozen ATPLs and served their SH type freeze. Most of these guys want to move to LH asap, so it will be quite a while before there are any DEPs going to LH.
Of course this could change, but BALPA would not be pleased, nor would many current BA pilots. Thus far BA have been playing by the rules so I see no reason why that would change.