I don't see any suggestion that the thrower was British. It could perhaps be interpreted as the urine thrower and Cavendish being the same nationality, but does not say what that nationality is. However I think anybody reading that would understand that it is Froome and Cavendish who are compatriots.
From a newspaper this morning, reporting on the Tour de France:
"Britain's Chris Froome more than doubled his Tour de France lead after finishing second in yesterday's time trial, though stage 11 was overshadowed by a spectator throwing urine at compatriot Mark Cavendish"
Now I think what that's saying is that the person who threw the urine was British, though as far as I know the culprit has not been identified, so I assume it means that Froome is a compatriot of Cavendish.
It seems to me that this is actually an error. - one of those situations where the reader realises that the writer cannot possibly have meant what he actually said, and so has to guess from the context what he meant to say.
Is there a name for this? It's a bit reminiscent of an unconnected participle "Walking through the woods, the oak trees looked very impressive", but it's not that. Disconnected noun perhaps?