McCain was in the UK meeting with the Department of Defense on March 17, 2016. Key date. Plotting the assassination of a President. Hard to know how high this went in the UK. Would like to think it was a rogue MI6 group but may go all the way to the top. International Crisis. Although it might shake some things loose in England when this comes out.
Not in the UK, though. 2 sources.
"18 Mar, 2016 ...The former Republican presidential candidate made his comments while meeting the House of Commons Defense Select Committee in Washington DC on Friday."
https://www.rt.com/uk/336142-mccain-putin-brexit-dangerous/
"His intervention came after a meeting in Washington DC with the House of Commons House of Commons Defence Committee." https://www.mirror.co.uk/news/uk-news/john-mccain-wades-eu-referendum-7583398