I flew with BA in the 747 in 2004.
It departed T1 at LHR. (well that's where we checked in but were bussed to T4 following an unserviceability)
Anyway - it is a great experience and worth doing - but weigh up the pro's and cons:
1. We arrived just before the flying display had started. (but by the time we got out etc....)
2. The terminal is at the far western end of the runway.
3. You have to check in for return before the end of the display (but they did allow us to sit outside the terminal and watch)
All in all worth doing I can assure you!
