There are actually two types of supply. If you are travelling on an A320 (not all of them by any means), then you'll need a different adaptor.

See the following advice from the BA web site:

Currently we have two systems providing power 'at seat'.

The first is a DC based system. The system provides +15VDC and is capable of delivering 75 Watts of power via a hypertronics connector mounted at the seat.
The DC system is being installed on our longhaul fleet and is available on the flat bed First Class seat, on the flat bed Club World seat and on the World Traveller Plus seat. For information, our entire fleet of Boeing 747 aircraft (Jumbo Jets) have been installed, and all of our Boeing 777 aircraft installed.
Installation on our fleet of 767 longhaul aircraft started in January 2005 and is due for completion by end 2006.

To use the DC system, there are a number of adaptors available commercially that come supplied with the correct connector for the seat. The adaptor BA recommends, and one that has been tested for use on board aircraft, is the new DC to 110 VAC Inverter made by Teleadapt. British Airways sells this model on board in the 'Shopping the World' catalogue, under Longhaul routes. To access this website please click here .
The Teleadapt adaptor delivers 110V AC via a standard 2 or 3 pin US or European socket making it universal. The Teleadapt unit allows the customer to plug in their standard lead.
Please note: The GD Empower system is identical to the 15VDC system that British Airways has fitted on its longhaul aircraft. If an adaptor is Empower compatible it will work on our 747 and 777 aircraft.

The second is an AC based system. We have fitted 10 of our new in-service A320 aircraft with this AC based Next Generation PC power system. This system features a US/Euro style 2/3 pin socket delivering 110V AC fitted to all Club Europe seats. This system allows the customer to plug straight in without the need for special adaptors. This system can also deliver a maximum of 75 Watts power. (See below attached drawing for socket information).

These powerports are not currently available in our World Traveller / Economy cabins on British Airways.

And here is the URL for the AC based adaptor picture http://generic-uk-questions.custhelp...ted=1081865982