To be honest the Russians can do what they like in the UK and they know it. The UK has no retaliation which will make any difference. We will just look stupid for making further empty threats.
We can seize Russian assets in Britain.
In time of war that is perfectly acceptable. Since a military asset was deployed on foreign soil, if Russia cannot show this action to have been committed by rogue agent or foreign power; then, in international law, that is an act of war that constitutes a declaration of war by the Russians (N.B. it's a declaration of war by the Russians, not by us!). We are entitled to at least temporarily treat Russia as a hostile power. We can legitimately seize assets and, even without trial, we can inter all citizens of hostile powers.
I'm not advocating doing these just yet (particularly not the latter), but I think they are a couple of things that may make a difference to Russia. To say that "The UK has no retaliation which will make any difference." is patently not correct.