when will the gb be playing away at the london olympics?
Depends who they are drawn against. Just because we are the host, we are not necessarily guaranteed to be the designated 'home' team for every match we play. This is especially so in the knockout stages, where the 'home' and 'away' teams are decided when the tournament draw is made - months before anybody knows which teams will be competing these matches.
In the Rugby World Cup final, France were designated the 'home' team, but chose to wear their away kit so that New Zealand could wear their home kit, being the host. I don't think this kind of respectful gesture seems to exist in modern day football.