Do you mean that you didn't want to tell them which way you had voted or you told them and were considered unsuitable for the sample, LH?
I told them, and the survey jumped to "On a different topic" with two more questions, and then end.
I've been a YouGov-er since 2000 and whenever (I won't say it paid for the extension, but certainly some nice scotch
) and I've got used to their ways. (I've also met their consultants in the course of my work). This was definitely a trapdoor question. It was specifically linked to my location, as it knew the candidates in 2015. Indeed the entire survey was geocentric, as it was careful to check my address was the same as their records before it started. The reason being it gave the names of candidates standing, as well as the party.
(It I were putting that survey together, I'd note that a lot of people haven't a fucking clue who the candidate they are voting for is. They just know it'll be "Labour", "Conservative" etc).
Now I'm not ashamed to say my answer was "Green party".
So the question is why did they not want a Green-to-Labour voters responses ?
They've already captured my intention (definitely will vote, Labour), so that part of the survey is accurate.