I'm getting hits from people Googling things of this nature, so let me make this easy for you:
Who was the British man who guest starred on Castle last night?
Well, there were two. Sort of. The actually-British older man was Charles Shaughnessy. (He played the suspect who worked at the consulate.) But you probably mean the Scotland Yard detective. That was Brett Tucker. Yes, he's actually Australian. Which brings us to . . .
Castle Scotland Yard Australian accent
Indeed, Googler, indeed. Tucker is Australian, but the character, Colin Hunt, was definitely supposed to be British. He said he grew up in working-class London and later went to Eton (on scholarship) and tried to fit in. My guess is that the accent was an Australian trying to sound like a Londoner trying to sound posh. It wasn't . . . an entirely successful attempt. I actually thought his Brit-trying-to-sound-American accent when he was impersonating Jack Bauer (did you catch that?) was better. Or he can keep standing around in a towel, and I'll stop criticizing his accent, promise.