I know there is a trick where you use 3.3ohm resistor that people use for aftermarket steering wheels. Did you have a look under your seat or the passenger seat at the modules and wires ?
I've just found this I never knew you could do this with the Subaru after 2001, but might help you if you want to try it
Also came across this not sure if you can get them in the UK or how well they work
http://www.rallysportdirect.com/Splash-Air-Bag-Light-Fix-Subaru-Mitsubishi