Not sure it's as simple as OH NO REFORM ARE GONNA BE AN ELECTORAL FORCE.
Firstly I doubt they'll win a seat, not sure where they're close. Clacton, somewhere in the north?
Secondly, what will the Conservatives be in 2029? I can only imagine on their second leader who will be a loon or an utter nobody. If it's a loon then that's Reforms purpose neutered. If it's a nobody then yes Reform may well start making in roads.
Thirdly the Tories main goal is to win elections. They will have to tackle Reform somehow. I doubt a party merger will EVER happen. Tories are too proud. I can't imagine the Tories standing aside at constituencies winnable by Reform, but if they do that gives Reform a tremendous amount of power and sway. The UK can tolerate the right, they can tolerate Boris, they couldn't tolerate Liz. Elections are won from the middle (2019 being a rare exception, but of course the Overton Window has been shifted that way). Reform are far from the middle, much like Corbyn was far from the middle. The Tories will eventually realise they need a centrist leader again, and dump Reform.
Reform will flounder under FPTP. At best for them, a weak Lib Dem style showing.
I'm not especially concerned by them and actually welcome their presence. The left has been split for too long, the right split is well overdue.
Besides 5-10 years of Labour in power will hopefully shift the centre point of British politics. This last point is very hopeful and ignores multiple outcomes, nuances, and unforeseen changes of dynamics.