Is your suggestion that the Conservatives would have got a bigger majority? As it was they had a net gain of 24 seats as well as increasing the number of votes cast for them. Traditionally, sitting Prime Ministers see a reduction in both - the last election where the PM saw an increase in seats was 1983, following the Falklands.
UKIP actually lost one of the seats it held, their vote was spread across the country, taking the role traditionally adopted by the LibDems (the non-Tory/ Labour vote), who imploded following their participation in the Coalition. It wouldn't surprise me at all if Reform performed rather similarly to UKIP in 2015, 13% of the vote and 1 seat. Even the sole victor, Carswell, had his majority squeezed (just over 3,000 when it had previously been over 10,000).
What happened next was that the significant number of 'leavers' on the Conservative back benches 'reminded' Dave that he had put an 'in-out' referendum in the manifesto, which he probably didn't need to do, he felt he had to honour it, Remain put forward a disorganised campaign and were beaten, not helped by a certain Mr Johnson who saw his opportunity for glory, and we know what happened next!