...for impersonating a police officer. Apparently he was dressed as a "plain clothes policeman".

Can someone explain to me how that makes sense?

From the BBC news story:

"Scotland Yard refused to comment on reports the man had passed through a security check at the castle by using the name of a senior detective currently heading a high profile investigation."

So he didn't need to be wearing a uniform.

Even more interesting because impersonation isn't even an arrestable offence...

edit: If however they were unsatisfied with the name or address they were given they could arrest him until his identity was checked out. More likely they arrested him under terrorism legislation.