Are actors taught to roll their "r"'s?

Many actors are taught to enunciate clearly, perhaps to ensure their lines are heard above a snuffling coughing audience, or to counter unpredictable acoustics in some theatres.

I'm sure that some actors who had trained for the stage would have then applied stage techniques to the screen, where close mics and sound stages would have made those old tricks unnecessary.
It's possible that the rolling "r" would be one of those tricks.

It really depends where the actors are from. Some drama schools would teach it, others would not. But a great many people in England roll their r's since birth, and I am not sure about Warwick, but it is possible that Jeremy knew how to roll his r's a long time ago.

As for me, I can easily roll an 'r' as can most of my Kiwi friends. So, no need to learn it in drama school.

The rolled or trilled R in question is a form of classical English pronunciation.

Before Queen Elizabeth II modernised her pronunciation, the trilled R was quite commonly spoken. In Britain, even to this day, dramas schools teach the trilled R as part of their voice exercises, and some choirs will not admit someone if they cannot rolled their Rs. Mr Brett's trilled R was very much in keeping with the character and period of SH.