If the Speaker is seeking to continue in the position from the previous Parliament, then he or she runs as the Speaker rather than for a party (since the Political Parties, Elections and Referendums Act  listed as "The Speaker seeking re-election"). Major parties don't run candidates against them, so there is not usually any meaningful opposition to beat.
If you instead mean MPs who run normally, then take up the position of the Speaker, the role is an elected one, even if historically a consensus on a single candidate happened before the actual election. As such, a prospective MP could only announce that they were willing to be nominated, and there is no formal requirement for them to do so.
It's worth noting that while the Speaker (and his or her deputies) does not normally vote, doing so only to break a tie, they can hold considerable soft power, Particularly in a hung parliament, as currently exists. As such a constituent with personal issues might find that the Speaker is better able to help them than a junior backbencher would be.