A quick skim of the list of members in Hansard shows that there is no consistency in how it refers to politicians – some are Ms Jane Smith, others are merely John Brown.
My understanding – I welcome corrections! – is that this is ultimately personal choice. MPs are asked to choose how they are described in Hansard, with the option for a title. (I am not sure quite how this process works, but I assume there is a form; there always is.) This decision eventually percolates through to all of the data produced by Parliament. Of course, “personal choice” might just be “whatever they [or their assistant] happened to think was expected when filling out the form”, rather than a conscious and deliberate choice.
So, what do people do? A 2010 Commons factsheet says vaguely that “A few Members of both sexes have requested that no title be used (e.g. Jennifer Jones MP” but a cursory glance down the list shows it’s more than “a few”. It turns out the full data is available from data.parliament.uk (as a big blob of XML) and so we can actually do some stats on this.
For current MPs, 145 of 650 have a preferred title (based on current data not past preferences). 33 Sir, 6 Dame, 17 Dr, 7 Ms, 11 Mrs, 71 Mr. Overall, 78% of MPs do not have a preferred title.
Of those, 44 are Labour, 93 Conservative, 3 LD, 2 SNP, 2 DUP, 1 Independent. So 17% of Labour MPs have a preferred title and 29% of Conservatives. Split by gender, 15% of women (32 of 209) list a title, versus 25% of men (113 of 441).
Of course, in some circumstances you don’t really have a choice – it would be a bit odd to say “I’d rather not be Sir X” once you’ve accepted a knighthood. Omitting anyone who’s a knight or dame, it becomes 20% of Conservatives & 15% of Labour having preferred titles, & overall 20% of men and 13% of women. The general proportions are broadly the same but the Labour-Conservative gap has narrowed a bit.
Doctors are an interesting question. Some PhD’ed MPs make a point of using their doctorate, but many others don’t. (They are in good company if so – the world’s most prominent doctorate-holding politician doesn’t, either). A couple of years ago, Chris Brooke tried to track down every current MP with a PhD. I took his list (with post-2017 updates), and a paper in the BMJ listing new medical MPs after the last election, and pulled together a total of 31 MPs who could be “Dr” – 21 have PhDs (or similar), 10 are medical doctors of various forms. (I have counted one with a D.Clin.Psy as “medical” rather than “like a PhD”). We’ve seen that only 17 people list themselves as Dr – who are they?
It turns out that every medical doctor uses “Dr” as their title, but only a third of PhDs (7 of 21). Two of the PhDs are “Sir”, but didn’t appear to use the title before getting knighthoods, and one sticks firmly to “Mr”; the rest are blank.
Across the parties, the Conservatives have six medics (all Dr) and 11 PhDs (three Dr); Labour have two medics (all Dr) and eight PhDs (four Dr). Not really enough to say anything confident about the difference between the parties.
Lastly, there’s the question of the change over time. Interestingly Paul Seaward noted that in the 1990s, the trend was for new doctoral MPs to use “Dr” for a few months and then quietly drop it.
The raw XML includes a note of the change of style by date since c. 2010 (presumably so that you can check you’re using a time-appropriate form if needed). It’s a bit noisy because it seems to have a lot of back-and-forth changes around election dates, which probably hints at changes not purely initiated by the Members themselves. Given this complicating the data I’d be cautious about drawing any conclusions from it without much more careful examination, but perhaps in a few years time we can start saying things about whether Members’ titles are indeed becoming gradually less common, or if it turns out that mostly not using them is a fashion that comes and goes…