Cheapest I can see is AXA for £48 from Amazon, IIRC B&M are £££, anything else?

B+M are just over £80 from Germany. I’ve got one and I just cannot fault it for quality, as a light or for USB charging. Mines hooked up to a rear light with a bake light too 🙂

EDIT
Just looked at the AXA. A couple of things spring to mind:
5v 0.5a USB seems a bit lightweight. Will it charge things or just keep them topped up? B+M has a cache battery such smooths the power too.
USB port on the light is neat but will it be as reliable and waterproof as the B+M version on the bars? I route mine into my bar bag and it’s pretty much 100% waterproof.

I already have a B&M light, it is v good. I was thinking of a £25 charger on ebay but I want a light for my Brompton so might as well try the AXA I think – I just want to charge a phone for dicking around with in the evenings., maybe daytime gps.

If you have an iPhone you need a regulated power supply that provides a constant voltage or it just won’t charge. I don’t know if that it the same for other phones but it may well be. The internal cache battery and electronics provide that regulated supply.

Considered the AXA but ended up getting a seperate charger, a B+M USB-Werk, £50 from one of the German MO places. 0.5ah 5V std USB output with a tiny cache cell to smooth out output.
The good thing is the cache cell meaning the Garmin charges continually but I think an Iphone would need a larger cell adding as the phone power draw (1ah/5V I think) will drain the USB-Werk faster than the cache charges. Will be same for the AXA I think, you’ll need a cell that can output the same as the phone draws.

I have the Axa light on my tourer, it will charge an iPhone and a Garmin Edge (actually charge rather than just keep them running).
The light itself is good; the bracket to fix it to the fork crown is very bulky though – I had to make an extra bracket to fit mine, might be awkward on a Brompton.

Ta, it’s be going on the tourer, the B&M going on the Brommie.

iPhone? Moi?