Since the annexation/secession of Crimea, the UK and other EU states already have sanctions in place against Russia, and Russia has been kicked out of what was the G8 (now G7 again). What more do you think people should be asking the government to do about Russia's activities?
The Stop the War protests involved people in the UK protesting about action or intended action by the UK government. Since we all know that the Russian government would pay absolutely no attention to a bunch of foreigners protesting about their actions in a third party country, I'm not really sure what the point would be.
Finally, why fix on one particular month of civilian deaths, given that this is part of a series of overlapping, interconnected wars that have been going on for about 15 years now? While Russia is doing an excellent impression of not giving a toss about civilians in Syria (as in Chechnya), they are hardly uniquely bad. If we're going to protest against intervening states killing large numbers of civilians for no good reason (or failing to protect civilians they claimed to be helping, which is just as criminal), I'll get to Russia after I'm done with the UK, France, the US, Israel, Iran, and Saudi Arabia.