The point where Russia might start using tactical nukes, is when Ukraine drives into Russia propper with massed forces. It is simply a whole different matter, diplomaticly atleast, if you nuke youre own country and in supposed self defense or atleast against what could be construed as an existential threat. Every country afcourse has the right to nuke itself, there is few to argue against that. Typically nuclear powers will abhor the offensive use of nukes in any scenario and especially in one where they have no interrest in, but by doctrine accept that they have a defensive purpose against a existential threat (at which point you dont wait to use them by the time they are at the gates of Moscow, or the red square itself, or inside the Kremlin) and that besides this their purposse is also to act as a detterent to such a existential threat. So even as a point to make, Russia would want to confirm that nukes are valid as a detterent to Ukraine pushing army's into Russia propper, probaly atleast use a nuke to make a point, and few Nuclear powers would want to deny that nukes can validly be used in this fashion or they would feel having nukes is kinda pointless. it would probably be overkill, but merely meant symbolicly anyway, and cerntaintly give pause to it's enemy's as how to go further.
All this is rather frustrating, it means that beyond Ukraine Russia is kinda untouchable, they have this line of safety behind which they can retreat. It's extremely heartbreaking knowing that Russia likes to deport and abduct Ukranians to Russia including children, and there is few in terms of leverage to be gained by Ukraine as to ever get those people back.
I'm glad btw. that the UK gives such strong support to Ukraine, and that this is wholeheartedly supported by it's citizins. Even when reflecting on the break in Europe that was Brexit, it's good to understand that we remain united versus this kind of dirtbags, if even that the Uk gives some further stimulation to France and especially Germany to do more, which can make us in part reflect on what it means to take a Leadership role in Europe in this context. I think it's a very positive development for the reputation and stature of the UK within Europe.