That Corbyn fella...

I get that, but they can't compel a country to join. I think Stop the War are ignoring the recorded desire of the Ukrainian government to join NATO (and indeed the EU).

I don't think they're ignoring it. They're taking a pretty firm position: that Ukrainian Nato membership should not be something the West should support as it's (in their estimation) more likely to result in war/further conflict than peace. Whether they're right or wrong is up for debate.

There was an edit after I responded, so I'll add to this:

Isn't the Stop the War position effectively ignore the democratic will of the Ukrainian people?

Yes. They state: "It has also declared that Ukraine has a “sovereign right” to join NATO, when no such right exists to join it or any other military alliance." They're saying membership to and international organisation is not something one has a democratic right to. If every country that wanted to join something only needed to express a democratic desire to do so things like Nato (and the EU) would look very, very, different.

If Ukrainians stopped electing governments that seek this goal, Russia wouldn't be threatening to invade.

One could also argue that if Russia hadn't already unlawfully invaded Ukraine (Crimea), annexed part of its territory and kept threatening to invade again with massive and provocative troop deployments, the people of Ukraine wouldn't be quite so keen about joining a military alliance, the primary purpose of which is to protect its members against Russian military aggression.