Politics play a huge part in making the idea unworkable. The ruling bodies in each country, the ones who we wonder what they really do, are unlikely to cede another justification for their existence. It is impossible to see the FA or SFA voting for the right to cease existence. Meanwhile money, as always, is an issue over which there are bound to be irreconcilable differences. The modern game has developed into a club-dominated beast.
International football is in terminal decline.
Big clubs fail to see the point of releasing players for friendlies. It's unlikely that would change with a GB team. The truth is that if we're to see a convergence of British football, it is more likely to come on the club scene. Celtic and Rangers are looking to the Atlantic League but if offered the chance to join the Premiership then they'd be there like a flash. The odds of a Great Britain team coming together lessen as the years pass.