Last time the requirements started the same as this (in fact it was Airbus, 747 and 777 only to start), then they needed 737 pilots. The requirements then gradually reduced as quality type rated people dried up.
Reading between the lines, the main reason for the TR requirement is actually that these pilots are needed for next summer but won't actually join the company until the start of the next financial year (Apr 2011), so it would seem to be a time restriction that is driving this. BA need the shortest conversion course length to get bums on seats asap after April 1st.
I think there is almost certainly going to be a need for more pilots later in the year, and I think BA will want to start recruiting integrated cadets as well, as they come in on a reduced pay scale for the first few years, plus it diversifies the age and experience profile which is a good thing.
As someone said, how many TR people are out there who actually want to join BA on shorthaul, and will pass the selection? Nobody really knows. I can see the requirements being relaxed to other types (including RAF) later in the year.
Don't despair yet if you don't have the right TR!