The situation is the same as in any of the other 649 constituencies that send a representative to the House of Commons, in that
if only one candidate is nominated, he is elected without a vote;
the elected MP is supposed to represent his constituents;
there is nothing in the law that stops any party from fielding a candidate in it;
any person who meets the conditions (age, number of nominees, citizenship, etc.) to stand for Parliament is allowed to stand in the constituency, and if he wishes he may promise to support everything that's in a specified party's manifesto even if he is not an official candidate for that party.
In any constituency it may happen that you don't like there being only a single candidate, or if there are two or more candidates you may not wish to vote for any of them. It may also happen in any constituency that you would like to vote for a candidate who stands on the manifesto of a certain political party but you can't because that party isn't fielding a candidate in that constituency.
If disenfranchisement meant "deprived of the opportunity to vote for a candidate who is standing on your preferred party's manifesto", then the answer to the question would be "yes". And it would be "yes" not only in the Speaker's constituency but in other constituencies too. For example, in the 2017 general election the Liberal Democrats chose not to field a candidate in two other English constituencies as well as the Speaker's; and there was an absence of Green and UKIP candidates in several constituencies in England, Scotland, Wales, and Northern Ireland, even though candidates from both the Green Party of England and Wales and UKIP stood in the Speaker's constituency. So if disenfranchisement meant what I just said, then a Labour supporter could be said to have been disenfranchised in the Speaker's constituency but not in Ilford North, whereas a Green supporter could be said to have been disenfranchised in Ilford North but not in the Speaker's constituency.
But disenfranchisement doesn't mean that. It exists when
there are two or more candidates and you don't have a vote;
there are two or more candidates and you are obliged to vote only for a given candidate, or to choose between voting for him or abstaining; or
there is only a single candidate because the law or the state has determined that that is how it will be, and you are only allowed to vote for that candidate, or to choose between voting for him and abstaining.
None of this means that your suggestion that the person elected Speaker should stand down and make way for a by-election is other than a good one. It could be argued that whoever is elected in your constituency should be tasked with representing his constituents by participating in debates in the Commons and by voting in Commons divisions where it is in his constituents' interests for him to do so. Of course the Speaker is only one person, so if he were able to vote whenever he chose then his vote would only affect a result in the event that equal numbers of members other than himself voted Aye and No, which is precisely the situation in which he does vote. But against that objection it could be pointed out that although he has a vote in those circumstances the precedent is for him to cast it according to Speaker Denison's Rule, not in the way that he feels best represents the interests of his constituents. In that sense I agree with the thrust of your question, and I would support there having to be a by-election.
But be aware that there is nothing in the law as it stands that prevents any party from fielding a candidate against the Speaker.