You will need letters from your treating specialists and current GP outlining the action and plan they are following to control your pain, but this will not guarantee other GP's will follow your continued care, if you know when and where you are going to, a bit of forward planning by ringing GP's in that area may help and save a lot of heartache.
I agree with Don plan ahead either with your current GP and when you know where you are going and for how long you can all up and see what you can organise.
Its a tricky situation as so many GP's are reluctant to prescribe controlled drugs, especially to new patients!
"Just thinkin' about tomorrow
Clears away the cobwebs and the sorrow 'til there's none
When I'm stuck with a day that's grey and lonely
I just stick out my chin and grin and say, oh
The sun will come out tomorrow
So you gotta hang on
'til tomorrow, come what may!"