The first past the post system makes it really difficult for new/different parties to win seats. Most people will vote for the same party they always do and even if they gain popularity then gaining enough in a constituency that is already established as a one/two party constituency is difficult. In 2015 UKIP got nearly 4m votes and it gave them 1 seat. The Lib Dems got 2.4m and 8 seats, the SNP got 1.4m and 56 seats. That was more than the SNP and Lib Dems combined but that got them 63 seats fewer.
I don't agree with anything Reform have to say but it is a nonsense system that gives so many votes so little representation in Parliament.
The biggest problem with Reform isn't whether or not they are taking votes off the Tories. Labour should be wondering why these people aren't voting for Labour. They are mostly traditional working class former Labour voters, even if they have voted for the Tories/UKIP more recently. This is what happens when Labour abandons the workers and moves to the right. Those voters aren't economically right-wing, they aren't wealthy, they aren't businesses they are people that would benefit most from left wing economics and they have only ended up over on the right because Labour have stopped talking about the negatives of immigration and how it impacts the people that have to compete for housing/jobs/public services with the additional people. It makes it too easy for people like Farage to stoke racism/hatred when all the opposition parties are saying there is no problem.