there is no need for booster seats.
available are special clips that you attach to the seat belt that makes it suitable for a child.
it brings the higher strap down to the right height for the child.
if you have children, just buy them one each.
found these on ebay
http://search.ebay.co.uk/seat-belt-clips_W0QQf cclZ1QQfclZ4QQfnuZ1QQfsooZ1QQfsopZ1
don't know where else you would get them except perhaps car shops or even mothercare type places.
I don't know how easy and quick these are to fit and remove - i,e. if they are portable and practical, but could be worth wading through to find one that would be suitable.
i daresay you could find something else that would do this job around the home - i know someone who carries those plastic ties that the police use as handcuffs, and a clipper to remove - takes seconds to put on and pull tight.
it would be nice if cab firms covered all eventualities, but quite simply they won't - it just isn't practical.
i like the inflatable idea though - i am sure taxis xould carry 3 of them with a fast inflater - thought would an inflatable rubber ring not be sufficient?