I thought about doing this, and read many many reviews of many different mounts, but the one thing that struck me about most of the reviews I read is that you're putting a potentially vital and expensive piece of kit on your bike, that could potentially bounce out, especially on a bike like a Brompton where bumps in the road tend to be a lot more noticeable. Unless your phone is waterproof, you're also entrusting the waterproof-ness of any cover they might offer.
When I crashed into the side of a car that pulled out on me in Regents Park in March, I seem to remember that my Garmin 820 didn't stay in it's mount and was picked up by a friendly cyclist that stopped to help, but wasn't damaged in the slightest, and perfectly recorded my incredibly fast stop. The phone in my pocket was also completely unharmed. Me, the bike, helmet, sunglasses, faired less well. Apparently 20mph into the side of a car can bend a Brompton frame, as well as the front wheel.
Depending on what you need your phone on the bike for, and if your budget stretches, maybe a separate bike computer is actually a better bet, simply because they're designed to be there and put up with some rough terrain and can put up with rain. Then you can keep your phone out of harms way.
Also, as much as certain apps for navigation on phones are great, etc., I do wonder about having a phone on a bike, from the perspective of so many drivers, pedestrians, etc., with their faces stuffed into their phones while using the streets in London, paying no attention to anything around themselves, that as cyclists, we should at least be setting some kind of example.