I'd like to know the answer to this as well. Got an England save going at the moment and just realised in 2016 GB didn't enter a side for the Olympic Games. I know they seldom do in real life but on FM 2012 I managed them for the London games and it was fun. Would've like to have done the same in this year's addition; not sure if you could last year or not. Has anyone seen a GB team reappear at any point in the future?
EDIT: So I just had a look on Wiki, it's a little confusing but it looks like it's the qualification rules that prevent Great Britain from being in the tournament/on the game. Qualification slots are given to the top few teams from the Under 21 EURO championship from the year before, and because England, Scotland, Wales and N. Ireland all enter that with different teams there can be no qualification spot gained by Team GB - I think. Because they were hosts in 2012, though, a spot was reserved for a united GB side.