From my observations, Boris Johnson doesn't "respond" when he chooses not to - he merely talks his way over questions that he doesn't like, hardly drawing breath and saying the same things over and over again, thus preventing the interviewer from seeking more clarity.
I only saw the last few minutes of the Marr interview, which consisted of Johnson's usual ramblings. However, during the Johnson/Corbyn questions to the leaders with Julie Etchingham, he kept on and on repeating "get Brexit done", the deal is "oven ready", "unleashing Britain's potential" - etc. etc. Just a roll call of slogans and no proper responses to any questions relating to other important issues. He also overran his time on every question, completely ignoring Etchingham's request for him to stop, and thus denying Corbyn his proper allocation of time. Perhaps Marr was mindful of this and, in order to obstruct these sorts of avoidance techniques, played Johnson at his own game - perhaps not the best tactic since it inevitably invites accusations of him "not letting Boris speak". Let's hope that Andrew Neil, who is a much tougher and more effective interviewer, has a chance to question Johnson - and that he is as tough on him as he is on other interviewees (bearing in mind that he has strong right wing links).
Johnson and other Conservative dignitaries thought it right and appropriate to interrupt election campaigning to celebrate the unveiling of a statue of Nancy Astor. Astor may have been a trailblazer for women in politics but she was also a notorious anti-semite who once said Hitler was a:
"welcome solution to the "world problem" of Jews." and, in an exchange with an MP, Alan Graham, said:
"Only a Jew like you would dare to be rude to me".
It has been said she was expressing views that were commonly held in Britain at that time, particularly in the aristocratic classes who saw fascism as wholly preferable to their perception of a communist threat, fascism being more in line with and protective of their own interests. Whether that excuses what has been described as her "virulent racism" is, I think, debatable but it feels very uncomfortable to me to be honouring someone who propounded such dreadful views - unless, at least, a comment underlining her fascist leanings and her unpalatable views on race and religion is included on the accompanying plaque.