Personally I consider myself a pacifist in all but very extreme circumstances. On the Syria situation, if we'd gone in what would our agenda be? Daesh are monsters, that's obvious. Assad is also a monster, that's also obvious. But the assortment of rebel groups in Syria aren't exactly wonderful people who embrace liberal western values either. Assad may have used chemical weapons against civilians and rebels, but so have Daesh and so have some of those rebel groups. The simplistic answer is that Daesh are the worst of the worst, so western powers could target them... but that would strengthen Assad (who is already propped up by Putin). So do we also enforce regime change and help those rebel groups overthrow Assad? That would further provoke Putin and besides, regime change worked so well in Afghanistan, Iraq and Libya didn't it? I don't have the answer to the Syria crisis, but crucially neither does anyone else- including Trump/May/the UN. Any intervention by western powers would create a quagmire and worsen the already disgraceful humanitarian crisis. Besides, do Britain or the USA have any moral authority to intervene after the Iraq debacle? Doubtful.
By the way, I haven't heard any voices from the Stop The War coalition coming out in favour of Russia's involvement in Syria so I'm not sure where you've got that from. What would you want to see from those groups and what would it achieve? Absolutely nothing, I imagine. They campaign against UK/USA involvement because that's where their sphere of influence is.
The "war footing" I refer to is the eastward expansion of NATO towards Russia's western border since the collapse of the Soviet Union, along with Trump's insistence that European members of NATO increase military spending and presence in that area. Putin is insane, that's as clear as day, so ramping up the pressure is only going to provoke Russia to respond in kind, hence the invasion of Ukraine and annexation of Crimea.
I'm certainly no Putin apologist, but our current strategy hasn't been working so to assume it suddenly will start to work is frankly ridiculous.
Yes, Corbyn could have condemned the Salisbury attack more vehemently, and probably should have done- but the fact he hasn't doesn't make him a Putin puppet. Putin is ideologically pretty much the opposite of Corbyn, so to draw that parallel (like the right-wing press have done) is absurd. Much of the media coverage of Russia both in the UK and USA seems to have forgotten that Russia isn't the Soviet Union any more, so the Red Scare and neo-McCarthyism is not only misplaced but is actually harmful.
Putin is a menace and a despot, but the people suffering most aren't us in the UK (apart from the Skripals et al, of course) but the Russian people:- particularly the minorities who have been targeted by the authorities there such as the LGBT+ community.
One possible approach might be to target the property investments of the oligarchs in London. All those luxury homes that stand empty, yet magically increasing in value at a far greater rate than any savings account? Enact a law allowing the seizure of those properties, and/or ban Russians with links to Putin from making further purchases. That would probably have far more of an impact than expelling diplomats. Though May (or whoever comes after) is unlikely to want to do anything to cause the gigantic pyramid scheme that is the property market to come crashing down.