There must be a big commercial opportunity for the likes of Boots to sell 100 ml bottles of their own brand liquids at airports and elsewhere.

As I've reported before, Tesco to the rescue! They sell 100 ml bottles of their own brand single solution. (Not sure if the OP uses single solution.)

And to say that the 120 ml bottle was 'stolen' is quite wrong. The rules may be stupid, but 'rules is rules' and a 120 ml bottle, even with only 5 ml of liquid, breaches the 100 ml limit. Sadly, ignorance is no defence.