In the U.S., workers are entitled to choose union representation by filing a petition with the National Labor Relations Board. If the employer refuses to recognize the union on the basis of a majority showing of interest, the NLRB will schedule an election. If the majority of workers vote for the union, it is certified, and the employer is legally obligated to bargain. Virtually every democracy has some mechanism for certifying unions on the basis of majority support.
Its the same in the UK but the company and union will work together - not sure what your point is.
I have worked on a successful UK recognition campaign BTW
The point is that the process the person you’re responding to is saying also applies if the company is not willing to work with the union. You did ask how else a union would organize without being blessed by corporate as this one is.