Of course you can compare on a sliding scale.
I don't agree with your last assumption for various reasons, in part because the reason the Unions are objecting to this is that, demonstrably, a great deal of their members don't have enough interest in what the Union leaders want to strike about.
Also, you can make "common interests" as wide as you like and then it also applies to the nation as a whole. All of us (presumably) want the UK to succeed and conditions to continue improving, so we have a common interest in the right decisions being made for the country. That there remains a strong amount of apathy most of the time doesn't invalidate this. The common interest of the Unions may be more narrow but it doesn't mean that there can be no comparison to referenda.
At any rate, I'm in two minds about these reforms, as hopefully can be seen above. On the one hand, it seems fundamentally wrong to make Union democracy so much more stringent than the democracy that really matters, so you have to suspect that it's an ideologically-driven "attack" from the Tories. On the other hand, the Unions lately are not doing the job they should be doing and have strayed too far into trying to wage a political war. Whilst they have been unsuccessful, perhaps some measure of reform to the Union system can help to protect them against the more militant parts of Unions.
Anyway, we'll see if this passes and how Unions respond.