Note: Keep in mind that phones are hard to read in strong direct sunlight, can overheat easily and switch off. And they are difficult to manipulate with gloves.
The bracket for handlebar mounting p/n 61712991100 at £61, then the case for the phone into which the bracket clips is about £35 (different part numbers for different phones).
Plenty of third party provide phone mounts, and using a GPS mount with some RAM hardware is probably the better option as it moves the phone higher in the field of vision. A quadlock on a RAM ball is quite practical for example. Or a X-Grip which also mount with a RAM ball.