Question that’s been triggered by the redistribution and “deselection”.
I understand that the map has been re-jigged so there is much angst amongst the sitting MPs about whether they will get re-nominated.
But in an ordinary election, when there hasn’t been a re-drawing of boundaries, how does the nomination process work?
Does a sitting MP automatically get re-nominated by the party, or can someone challenge the MP for the nomination?
I don’t believe re-nomination is automatic. I think the MP can be challenged but don’t quote me on that. Constituency parties usually work fairly closely with the sitting MP so challenges are relatively uncommon, or are usually doomed to fail. With redistribution of seats and “entryism” within the Labour Party things on the deselection front are more interesting than normal.
“entryism” 
Sorry, the allegation that the Labour Party has been infiltrated by hard left types. These new members of the Labour Party are accused of infiltrating and influencing the Party at local constituency level. MP’s are threatened with deselection. As yet these are just allegations, I don’t believe a sitting Labour MP has been deselected recently.
To go back to the OP, each party does it their own way. The presumption is that a sitting MP will be “adopted*” again unless they choose to stand down or have seriously annoyed their constituency party in some way: sometimes because of something they have done politically or personally, but sometimes because of something they haven’t done (i.e., being a lazy so-and-so). They will usually know well in advance if they’re in that sort of trouble - it’s the job of their constituency chairman and executive committee to let them know if necessary.
Each party may have its own rules as to how members of a local party can set in motion the process of a new selection procedure if the constituency officers and executive don’t want to; and it’s a regular issue in the Labour Party as to whether sitting MPs should be automatically required to go through a full re-selection process.
http://www.britpolitics.co.uk/a-level-uk-political-party-parliamentary-candidates
*The formal kick-off point for a General Election campaign is the “adoption meeting” where a constituency party formally agrees to nominate their candidate, but this is usually no more than a formality and a bit of a legal manoeuvre (to separate the campaign expenses from the regular local party business).
Quite different than here in Canada then. Here, as far as I know all parties require a formal nomination meeting prior to an election, and anyone who meets the number of signatures can stand for the party’s nomination. It’s not tied to the local party association executive at all, although of course a popular MP will not likely be challenged.
Party HQ can sometimes get rid of them, known as withdrawing the whip.
Local party members can do so in some parties, but not all. They can in the LibDems, which is ironic as the SDP was largely formed because right-wing Labour MPs were alarmed at having to actually be popular with their constituency parties, and so left to form their own party.
This sometimes leads to party members standing against their own party.
In 2005, Peter Law, who was a Labour member of the Welsh Assembly, beat Labour in the Blaenau Gwent constituency after Labour used an all-women shortlist to put a head office stooge in place as the local candidate.
In 2010 Tories in Buckingham founded a breakaway party to try and get rid of their MP, John Bercow, currently Speaker, due to his personal unpopularity. They came second, none of the main parties having a candidate standing against the Speaker.
Central Party figures can also impose candidates against the constituency advise.
Depends what you mean by ‘impose’. The Conservatives and the Liberal Democrats require their local constituency associations to pick candidates from a national list of approved candidates. So anyone that the national HQ doesn’t like will have already been weeded out. In contrast, the Labour party doesn’t have a list of approved candidates, but does place other restrictions on the choice, most obviously by requiring some constituencies to use women-only shortlists.
Talk of imposing candidates instead arises when the selection has to be made quickly. Such as with by-elections or when no candidate is in place when a general election is called. In such situations, the selection process is shortened, usually in ways that give the national HQ much greater influence. This is particularly true for by-elections, because the national HQs understandably want candidates who will look good under the additional media scrutiny.
Thanks for that info, APB - much more centralized under party control than is the case in Canada.
AFAIK it’s intended to weed out the flakes, crooks, downright incompetents and people with embarrassing pasts that, as I understand it, the American system expects to expose through the primary system*, and to identify talents that still need some mentoring and training to become competent/credible candidates. It isn’t necessarily intended to ensure total ideological conformity with the leadership of the moment (you certainly wouldn’t get that in the LibDems, for a start).
*[tongue slightly in cheek]