For example why did radio presenters roll the r on the BBC before the war and not after? Why did Brecht roll the r extensively? Why did Hitler roll the r extensively? My perspective is from the performance point of view but I am looking desperately for linguistic texts sources as it seems a common phenomenon? Any articles in research or conference papers you could help me with?
The short answer to your question for both English and German is early twentieth century stage pronunciation, an artificial, overarticulated accent designed to project to the back rows of a theater before the routine use of microphones.
In a 1920s recording of John Gielgud reciting Othello’s speech from Act I, Sc. 3, the actor pronounces an alveolar trill [r] in reverent, rude, and greedy, more briefly in dearest and around, an alveolar tap [ɾ] in very, married, redemption, seriously, and in “linking r’s” in ever and her. The rest are either dropped or vocalized as typical with non-rhotic BE or the regular postalveolar approximate [ɹ̠].
This over the top style of acting, especially the highly affected treatment of vowels to evoke great pathos, is long gone, as Gielgud and Olivier recall “singing” Shakespeare’s blank verse, the former with no small measure of embarassment as he remembers earlier performances delivered in that style.
As the few instances in Gielgud’s speech suggest, the trilled allophone was never a regular feature of either older RP or the stage, but an occasional means of adding emphasis or stress. The word reverent, for example, comes quite early in the speech and functions almost like a drumroll.
The trilled r copied from the stage was not a constant among newscasters. Earlier Pathé newsreels, even those where a certain gravitas and dramatic delivery might be expected, such as the death of Queen Mary or the evacuation of Dunkirk, abound in tapped, but not trilled r’s. The first broadcast of the Queen’s Christmas message in 1957 offers another comparison: no trill and taps only on very (twice) and American.
In April 1898, a conference of germanists and theater directors gathered in Berlin under the direction of linguist Theodor Siebs to hammer out a unified pronunciation for the stage. The results were published later that year as Deutsche Bühnenausprache (German Stage Pronunciation) and remained authoritative until supplanted in the post-war period. It was heavily oriented toward the written language and a northern pronunciation, that is, how High German was spoken in areas where local dialects were traditionally Low German.
One southern feature, i.e., Austrobavarian, East Franconian, Allemanic, he did prescribe however was the alveolar trilled r, reduced in a few cases to the alveolar tap. The realizations of r most common today, the uvular trill [ʀ] and the voiced uvular fricative [ʁ], were not permitted in later editions of “the Siebs” until 1957.
In this brief speech of April 1939, Hitler closely follows Siebs pronunciation, the only trace of his native Austria perhaps a slightly darker l. Even in medial and final position, the r is articulated as an explosive trill.
There are very few recordings of Hitler’s normal speaking voice, but his accent shows far more traces of Austrobavarian than his speeches. In this recording, admittedly in poor quality, I heard only one alveolar trill, in Arbeiter ‘worker’. The rest are either vocalized or flapped. Other features are monothongization of the ei diphthong, Tag ‘day’ pronounced with a lenis d and g, final l umlauting the vowel in Beispiel ‘example’, though the l is still pronounced. Today, one would call this accent Standard High German with an Austrian coloring (Färbung).
When Bertold Brecht — who didn’t leave his native Bavaria until he was 27 — recorded his own poems, he used a modified Siebs pronunciation with far more Bavarian features, including the alveolar trill, which was part of his native dialect anyway. To hear the difference between this accent and and that prescribed by Siebs, compare Brecht reading his poem “An die Nachgeborenen” (To Those Born After) to the interpretation of the same poem by actor Klaus Kinsky.
For the two quoted speakers of German, dialect is an explanation. Brecht is Born in Augsburg (Bavaria) in an area where r's are rolled, and Brecht used Süddeutsche Umgangssprache (Southern colloquial German) quite consciously.
I am not so sure about Hitler who is born in Braunau (Austria) in an area where r's are rolled. But I suppose, he received some training on pronunciation.
R's are still rolled in Bavaria, when you listen to German speakers from Bavaria and Austria today you will notice it. Rolling the r's is a requirement for professional speakers in the official Bavarian radio network (Bayerischer Rundfunk).
There is an interesting discussion of Hitler's speech in Hubert's contribution to this thread: https://german.stackexchange.com/questions/40864/gab-es-einen-deutschen-posh-accent
What is significant is that Hitler went out of his way NOT to speak with an Austrian accent, but to emulate as well as he could the "deutsche Bühnensprache", one of the obvious features of which is (or rather was) the alveolar "r".