Originally Posted by
stifle
There are two possible obstacles. One is the purple suits do sometimes ask to see onward travel documents before allowing access to the buses, and the other is that this could not of course be done with checked luggage.
For the first obstable, if that is encountered when doing T3 to T5 (I must admit I haven't experienced teh bp check myself) then just say you have a connecting flight on BA and that you need to collect your bp at the service desks near T5 FCC. They should allow you to proceed.