I don't think I've ever heard the term Mr Prime Minister used informally in the UK. It is far far more common to hear and use Prime Minister when referring to or speaking at the Prime Minister. Using Mr/Madam etc. + [Title Name] is far far more common in the USA than the UK and I'd wager that the American precedent has been applied wrongly here. RavingWelsh (talk) 17:41, 22 November 2021 (UTC)
Sadly I myself can't edit it away, but there are a few websites even from the US acknowledging the convention that it is just 'Prime Minister' the confusion may be that Canada sometimes uses the term 'Mr Prime Minister' but can confirm from my experience it is almost never done here. RavingWelsh (talk) 20:15, 22 November 2021 (UTC)
Thanks! Went to check the Deputy PM page and have changed its style to be in line with this one too. RavingWelsh (talk) 20:24, 22 November 2021 (UTC)
The info box should be edited to change the appointer being listed as the Crown to either Elizabeth II or to The Sovereign linking to the British monarchy page. The Crown is analogous to the state in the Commonwealth realms and shouldn’t be used as shorthand for the King/Queen of the United Kingdom. 2A00:23C8:A2CD:D01:CAA:87C3:17DF:535B (talk) 20:39, 5 April 2022 (UTC)
The second paragraph says "It was announced by Prime Minister Boris Johnson on 31 January 2022 that the office of Prime Minister would be formally established" but that isn't what the cited article says. It says he's creating an "office of the prime minister" i.e. a group of civil servants led by a permanent secretary to support the prime minister.
The actual office of prime minister (not the office of the prime minister) remains as it's always been.