I've had my Brompton almost four and a half years now.I also have another folder and five road machines and I've tried a number of mirror types over the years includeing all those mentioned .The best mirror I have found for the Brompton , is the cheap(about $7.00) ones made by Bell and other companies that velcro on to the bar ends.They work very well once you get used to them and when it's time to fold they come right off in seconds and stick it in the bike bag,or just use the velcro strap to attach it some where on the frame.
One thing I do like about them is that you can adjust them easily with just a twist of the hand for a better view behind.Not to everyone's liking I'm sure but they work well for me and the streets i ride.