No, they are represented by the MP. They elect an MP not a party. If an MP changes party the electorate are not unrepresented.
I don't think that's correct in any meaningful practical sense rather than in a purely procedural manner.
The ballot paper lists both the name of the candidate and the party they are representing. While it is true that a person is elected we cannot know for certain whether individual voters are making their decision primarily (or exclusively) on the basis of the person, primarily (or exclusively) on the basis of the party that they are standing for or a mixture of both.
That said the lack of success of independent candidates, including those that may state no party affiliation on the ballot paper strongly suggests that the party, and not the person, is the primary motivator.
Think of it this way - do you think Ferrier would have been elected in 2019 if under her name on the ballot paper it didn't say 'The Scottish National Party candidate' (or similar wording)? I think we know the answer to that.
So there is a completely legitimate argument that when a person is elected that do so on the basis and understanding that they represent a particular party. So although constitutionally they cannot be kicked out of the commons if they are expelled from the party they were elected for, or cross the floor, there is a reasonable argument that their 'contract' with the voters no longer has legitimacy as one part of the reason they were elected (the party they stood for) is no longer valid. And the ability of an MP to represent their constituents is surely based on the legitimacy of their representation, both as an individual and on the basis of the party they stood on behalf of, both of which were on the ballot paper.
And I think the electors also recognise that the legitimacy is gone - on the basis that it is hard to think of an MP, elected as the candidate for one party subsequently standing with the same constituents (i.e. in the same constituency) being elected for another party (having crossed the floor) or as an independent. Perhaps there are examples, but I can't think of any recently.
If the electorate put great store on the person rather than the party then surely those individuals would romp home in the same constituency at the next election standing either for another party or as an independent.