I believe there needs to be at least a few Non-English players for it to be credible, Ben Flower and Regan Grace from Wales are the first two players I can think of, there is also Huddersfields young Irishman Ronan Michael but I'd be surprised to see him called up due to his inexperience.

Are there any decent Scottish players who could get a call up?

And therein lies the problem.
Right now, a Lions Tour is just England wearing a different badge.
Should we weaken the squad to make it less English ? I dont think so.

Players, whichever country they are from, have to be the best that can be found, who qualify as Great Britain representatives.
There should never be token players, just to "pretend".

Grace and Flower are no where near GB level and shouldnt be considered just because because the are Welsh. That would totally devalue the shirt.

To be honest i fail to see where any Welsh Scottish or Irish players would get anywhere near the side so i dont see the point of making it GB.

Well, a GB tour has greater status than an England tour - as already proven by it being the first item on the radio 2 sports news at 4 o'clock.

Danny Brough for the token Scotsman
Regan Grace for the token Welshman
Struggling here for the Irish, maybe George King?

Would people rather see representation from all Four Nations or the best team on the field?

I imagine if need be the Non-English players will start against Papua New Guinea and perhaps Tonga.

 Will there be any warm up games beforehand?

There wont be any non English players

Danny Brough may be the most in form British HB in SL at the moment but, if he couldnt get a gig when he was in his prime, he's got no chance now.
His kicking game is THE best in SL, although discipline can be an issue.

At the end of the day, it's just branding. I really don't see the need to get caught up in how many Welsh, Scottish and Irish passports we can find in a grandparent's drawer to justify the GB tag.

It's a GB tour, 'GB' is a name that the RFL believes is more recognisable to the Aussies (who will be buying most of the tickets given that GB is predominantly a touring side) and other than the most dyed-in-the-wool supporters on RLFANS, does anyone really care whether that are enough players to recreate a "walks into a bar" joke?

Just England in a different shirt. Just abandon the England team if that's the case.