I assume that you omitted the word 'voted' (3rd line)? In any event, whether the word is included or not, the statement is nonsense: The members of the Union vote in the leadership of their Union. How is that undemocratic?
It appears that you would deny individuals the right to withdraw their labour?
Not all the public are in favour of the strike, so it is a risky strategy if opposition MPs see it as courting favour with the electorate, hence the reason that Starmer has warned against them being seen on the picket line. Maybe those opposition MPs who defy Starmer on this matter are simply there because they believe in the RMT's case.
It appears that your version of democracy comprises a government - of which you approve - with no dissent permitted. Now that is a strange version of a free, democratic society.