In the UK, 'officially' nobody ever votes for a party. Votes are for individual candidates.
When I was younger ballot forms only bore the names of the candidates. (It was illegal for any reference to a political party to be shown on a ballot paper. It was expected that voters would ascertain the views of candidates, and any political allegiance, prior to entering the polling station). It was eventually accepted that some voters would find it useful to see the name of a candidate's political party (if any) shown alongside their name but the necessary change in the law only came about after a great deal of debate in Parliament.
Despite the change in the information available on ballot papers, the basic principle remains unchanged. You (officially) vote for a person, not for a party.
In practice, none of the major political parties would endorse any candidate who was also a member of a rival party. However there are people who do stand 'on different tickets' when there are no obvious conflicts of interest. For example, someone might stand on behalf of the Blogborough Residents Association Party in a local election but also be a Green Party candidate in an election for a wider area.
Chris