The UK can prevent a legal referendum, as that is where the constitutional power lies. An SNP referendum can have no legal sanction, but then supporters of the current arrangements are in no need to actually partake. That is ine complication.
Then there is the problem that the SNP has been in power for a long time and it’s record is rather mediocre, with some long running issues coming to fester.
Finally, because the decsion on what actually to do is not as straightforward as you seem to think, there is a small and rather unpopular party set up with the intention of chivvying the SNP leadership into an unsanctioned referendum. I think at this point, most of the received opinion is that this would be a bad idea.
Time will tell. I think at some point that referendum ('illegal' or not) will be held, and then what happens with respect to the outcome is going to be an interesting lesson in power politics.
After pushing through Brexit and involuntarily causing Scotland to drop out of the EU that won't go down well, I guarantee it.
Calling it illegal is already pushing it, and will likely result in more votes to secede rather than less. A couple of hundred years of Scottish history are testimony to that being a country that you don't want to push around too much or it will surely backfire.
Unless you want to suggest that UK would go to war or try to economically strong-arm Scotland into staying in the Union but I don't think even Johnson and his merry band of robbers is going to go that far and without enforcement the term 'illegal' holds no power.
Then there is the problem that the SNP has been in power for a long time and it’s record is rather mediocre, with some long running issues coming to fester.
Finally, because the decsion on what actually to do is not as straightforward as you seem to think, there is a small and rather unpopular party set up with the intention of chivvying the SNP leadership into an unsanctioned referendum. I think at this point, most of the received opinion is that this would be a bad idea.