The British tactics were pretty much perfect. But basically because no other team wanted the race to end in a bunch sprint (as Cav would win that) the team had no help at all to bring back the breakaway group. Now, in the Tour de France with a 9 man team it is possible for one team to do all the work and still win. In the Olympics with a 5 man team that is impossible. Therefore the only chance was if one other nation had also decided they wanted a bunch sprint and thus helped the peloton to catch the breakaway (Germany only real chance of that) but they did not. Therefore, through no fault of their own (just the nuances of road cycling) Great Britain (Cav) had zero chance. That's how it works.