On the aircraft it's on the back near the rearmost door, both sides. The last 2 letters are on the 2 front landing gear cover doors, and the last 3 are right on top of the cockpit window. The number can also be found on a plaque type thing on the main galley, and probably on a bunch of equipment in the other galleys.

ftdashboard, lhr-lgw.co.uk, and the crew onboard can also help!