You really don't understand operations in war, do you Ian? Which is fair enough. But this stuff happens the whole time.
We're not talking about a regiment of soldiers here. We're talking about maybe 10 or so specially picked operatives who go in there to help the Ukrainians out tactically, spread a bit of chaos, potentially take out specific targets (not just human, but bridges etc to sow discord into the Russian offence). That's what happens in a war theatre. And as someone said before, I'd be surprised if there wasn't a few of them there already.
How to get them in there? You do realise that there's a load of Ukrainians pouring through the Polish border right now to escape. They would go through in the opposite direction. Simple really.
What does Salisbury have to do with it? That wasn't during war, and they did it really, really, really badly. You may have also noticed in the last few days, the Russians don't need an awful lot of excuse to do mad things either. In the unlikely event of a couple of special ops guys getting caught alive, they don't sit there and say "yeah, sorry guv, I'm special ops". They're unidentifiable.