My guess is that BA just wants to make sure that its customers check in at the brand-new Terminal E check-in hall, rather than getting dropped off at the D check-in hall which BA has used for ages.

I've walked from to D to E landside before, but that's not a recommended jaunt particularly with luggage, especially since one has to dash across a busy road to do it.

The walk from the new D-West gates that QR and BA have been using to customs and immigration at E is not great, either. It's long (and drab), but most folks who are veterans of LHR are probably used to long (and often drab) walks!

On a positive note, the new E check-in area is bright, airy, and spacious, which may be a welcome change from the congestion in the D check-in hall.