Rern/Klassikmann
Sorry, answered my own question. I had forgotten that the old PI I was trying the touchscreen on was actually an original RPiB+ (ie Pi 1B+). This obviously isn't up to the task of running the browser and so the option is hidden away (same on a RPi Zero W when I checked). On my Pi2/3's the option is available so I will switch the touchscreen to one of them.
Sorry, should have looked/ checked further before posting.
Chris