I've just tried this.
What I did was change every city in Wales, n. ireland, scotland and england to Great Britain. This way all the stadiums are in Great Britain so youth players with Great Britain as a nationality get produced in the 4 countries. To make sure there was a fair few players at the start for great britain I filtered out everyone with an ability of below 110 in Scotland, wales and n.ireland and 120 in england. Lowered the reputation of england, scotland, wales and n.ireland to what I thoguht would be reasonable so their grey players aren't unbelievable in the future.
Took a while but it seems to have worked, England's best player is Matty Fryatt but Great Britain are basically just England plus Craig Bellamy
Am running a game at the moment to see if they get entered into qualification for the Euros.