If you have Oneworld status (which I note that unfortunately the OP does not), you can select seats much as the same way that you can on BA.

Here's some photos and a report on the BA WTP product:

http://www.flyertalk.com/forum/trip-...eb-2010-a.html

And the Qantas PE product on the 747 (which is what you will be on going to Bangkok):

http://www.flyertalk.com/forum/trip-...y-pacific.html

Also, here are some previous threads on the matter:

http://www.flyertalk.com/forum/briti...m-economy.html

http://www.flyertalk.com/forum/briti...z-ba-v-qf.html

http://www.flyertalk.com/forum/briti...mium-econ.html