Ian, I use GPS constantly. Although it's on a TomTom not my phone.
It's absolutely essential. Here's a typical day :
Start at 5am
Wrexham to Barrow in Furness
Barrow in Furness to Hawick
Hawick to York
York to Hull
Hull to York
York to Wrexham
Finish at 9pm
With a total journey of just under 650 miles there's no time to get lost.