I asked first to show me that it is the case. Go on.
It can't be proved either way can it? Your premise is that Reform is offering what "voters want", but voters in this instance is some nebulous concept. Any political party puts forward its policies, by definition that policy can only be wanted by the person that votes for it. If you don't vote for a given party, then you do not want their policy offering.
I think your statement is really more that Reform is more popular than someone on the left may think it is, that could well be the case, but it is not the same argument as Reform are offering what the voters want, they are offering what certain voters want i.e. people who vote Reform.
If I say Labour are just offering "what the voters want" the exact same principle applies. They are not offering what voters want, they are offering what labour voters want.
You use the example that other parties are chasing Reform voters. They are, but they did not win Reform voters in local elections did they? This undermines your whole point. If Reform are offering what voters want and Labour basically offered the same on immigration (as you mention the immigration point), then why didn't people just vote Labour as they were offering what voters want according to your argument?
I am not denying that Reform are popular, I am simply saying that a blanket statement that they are offering what the voters want is far too broad to have any real meaning. They are offering certain policies that appeal to certain voters, just as any political party does. All you can prove after the number of votes counted is that more voters wanted what one party had to offer than the others, but that is not the same thing. Labour just won a by-election so by your definition, Labour is now offering what the voters want. No, Andy Burnham just resonated more in Makersfield and won an election.