I'm sure Brompton are overcharging but they occupy a particular market niche, which no-one's been able to break into for 30 years. And Bike4Work/Cyclescheme has pretty much set a £1000 price tag for a commuting bike, because it's not real money, and they're exploiting that fact. An M3L was £550 only 8 years ago and now it's nudging a grand.
When branding and an associated ecosystem are taken into account, market forces no longer work. Look at the markup on an iPhone.