A few thoughts on sanctions and other economic tools, lest anyone think that they are "soft":
There are steps that can be taken which will hurt Russia, and send a signal that such an outrageous act (invasion of a sovereign state) is completely unacceptable and utterly disproportionate (and, I write this as someone who has some sympathy for the Russian leaning "separatists" in eastern Ukraine; steps - such as on language recognition - could have been taken to address their sense of alienation and grievance - some of which were quite legitimate - ages ago).
Political: Visa bans on all government ministers, on all members of the Duma and Federation Council, all governors and office-holders in Russia’s regions, on all officials in the “power ministries” and security agencies, and on the 35 individuals named in Alexei Navalny’s list.
And on their spouses, siblings, offspring, parents and other associates.
Economic: Shut Russia out of the Swift banking system, and impose sanctions on - and freeze the assets of - any and all of their banks that have links to their government and trade in the west; Announce a sweeping program of asset freezes on these individuals, and on companies linked to the Kremlin or owned (even in part) by Kremlin cronies.
Delist these companies from the exchanges in London, Frankfurt, New York and other western financial centers. Stop trading their debt, bonds and other financial instruments.
Take serious steps (a medium to long term plan) to curb our dependency on Russian oil & gas.
This will also mean persuading our own electorates - yes, our Green leaning electorates - to slow down the pace of environmental change away from carbon; in other words, would I be prepared to pollute the environment - keep coal, for example - burning to ensure western homes remain heated while we reduce our dependence on Russian energy and simultaneously take around a decade to develop alternative sources of fuel? Yes, I would.
Our electorates need to be advised that actions have consequences and that if you own the action, you also own the consequence.
Diplomatic: Promote bare bones embassies, and shut Russian consular and "trade" missions. (Yes, that will be reciprocated passionately with vehement vengeance by the Russians).
Cultural: Remove Russia from the Eurovision song contest.
Refuse to play football matches in Russia, impose sanctions - and freeze assets of - on Russians owning media in the west, or on owning football clubs (Chelsea, anyone?) or the like.
I would even contemplate expelling Russia from FIFA, and from European football competitions, or boycotting them. Exclusion from international sports - soft power though they are - will hurt, in reputational terms.
Expel Russia from the Olympic Games.
Remove RT and other Russian propaganda organs from the airwaves in all countries that have regulated broadcast media. (Yes, this would mean reciprocity: The BBC and others would then be exelled from Russia. So be it).
Personally, since I do not (and would not) recommend military action, I would strongly argue for any and all economic (diplomatic, cultural, and political) steps to be taken - some of which would (no, will) hurt us - that will signal, strongly, our deep disapproval of this step.