YANBU that Labour can fix it, YABU to worry that it will lead to a far right government. Reform will crash and burn soon enough, I doubt they will ever be in a position where they can challenge for power. At best they might challenge the Lib Dems and SNP for third place and hope to get into a coalition.
Even if they did take power outright their support will quickly fade once people realise how terrible they are. Half their support will be appalled by the reality of deporting everyone who has migrated into this country since WWII and their descendants, and the other half will accept nothing less than that happening. So either way they'll destroy their own base and be unelectable in future.
I'm not sure how feasible it is to even deport so many people. One, where do you send them if they won't go voluntarily? A lot of them have British citizenship and nothing else. Other countries don't have to accept them and it's not as if Britain has an Empire where people can be forcibly deported to. If you can't deport them, what then - lock them up in concentration camps? It would be seriously unlikely that other countries would just sit by and let that happen. Even if there was no military intervention there would certainly be sanctions, and I doubt most Reform voters have the stomach to endure the kind of privations the North Korean public has suffered over the last few decades.
Two, how do you determine who is "British enough" to remain? Do we have a US-style "one drop" rule whereby anyone with any trace of foreign blood is removed? If someone has one great-grandparent who was an immigrant, but their other great-grandparents were British, do you remove them?
These questions and points may sound ludicrous or even offensive, but these are the realities of implementing the far-right policies that most (not all) Reform voters want. The point is, the practicalities haven't been thought through - or if they have, they've not been explained yet.
Someone will undoubtedly claim that Reform don't actually intend to do these things but that in itself would be another reason Reform are doomed - a lot of their supporters expect and demand it, and will move to a party further to the right if Reform don't meet their expectations.