Hi
If you want to run a laptop on those BA 747's you need a laptop power supply kit. Its NOT like the car and often differs between airlines.
I have one from a company called teleadapt.
Its basically a box that your laptop power supply plugs into and the teleadapt plugs into the seat power socket.
The power supplies such as teleadapt have different plugs normally to cater for the laptop puwer connectors which may differ.
If you buy one, its normally expensive and between £65.00 - £90.00 pounds. Also make sure your laptop connectors are correct, normally these power adapter kits tell you the models such as Sony, IBM etc.
Cheers
Sean