I asked the road safety officer at our local council (which is very very good at road safety stuff)!
He told me that Britax advise turning off airbags if there are children in the front seat, while the transport research laboratory advise that if the child is at least a foot (30cm) away from where the airbag discharges, then they would be OK. He said he would go along with what the trl advise.
Obviously, this only applies to front facing seats, rear facing should NEVER be used where there is an airbag.
As i said, our local road safety people are very good, and this bloke normally spends the first hour or so of his holidays at car hire places properly fixing car seats into hire cars for people. He regularly goes to baby and toddler groups for advice and will happily check people have thier seats in properly, he really knows his stuff.
Hope this helps.