Going back to the Sensor 2, some practical experience I've picked up that may be of use to others:
It's really helpful that the Sensor 2 can record mileage independently of a GPS "head" device, and that you don't have to remember to switch it on and off every time. What's more, it doesn't record you at 80mph when you've ridden to the station and got onto a train to get to work! (Because the wheel's not rotating.) However, there are issues:
* Once the phone and Sensor 2 are communicating, they don't seem to drop the connection, and the Sensor therefore stays awake as long as the two are in Bluetooth range of each other. This is bad for the batteries of both devices, and leads to the Sensor recording very long rides, during most of which you were not actually moving. If the Brompton lives by your desk at work, and on the train is also close to you, it may not go out of Bluetooth range all day.
* If you have a Touring Plus and Sensor 2 on the same bike, they don't communicate directly of course. However, Connect then doesn't seem to spot that two recorded rides are the same, and so does not deduplicate them - it has you doing two different rides at the same time, which is daft. As most of the logic must be in Connect, I can't really see why it couldn't do this, or allow the merging of activities.
* As I understand it, Connect should improve the calibration of a Sensor 2 using measurements from a GPS. This doesn't seem to happen with the Touring Plus, even though it's not obvious why it would depend on the Touring Plus and Sensor 2 connecting directly. This is probably linked to the previous point.
* As above, some Brompton folders have a three-speed rear hub and a front dynohub, so the Sensor strap doesn't fit either. A spare Cateye Nima light strap works well as an extension.
As I say, it does provide a means of recording commuting mileage without having constantly to switch a GPS on and off, but hope these points are of interest to anyone considering this approach.