According to AI:

Sanctions on Russia are considered legal because they are imposed by countries or international organizations in response to violations of international law, such as aggression or human rights abuses.

Sanctions against Russia were implemented following its invasion of Ukraine, which is viewed as a violation of international law.

The only problem with this argument is that the whole justification for sanctions has been based on carefully crafted falsehoods like those peddled by 'Ambassador Holland' who really should sack his speechwriters.

[https://www.gov.uk/government/speeches/russias-war-in-ukraine-has-been-built-on-a-foundation-of-falsehoods]

And is why the US sides with Russia in UN resolutions on Ukraine

[https://www.bbc.co.uk/news/articles/c7435pnle0go]

As Jacques Baud rightly points out in his book Covert Wars in Ukraine, the two stated aims of Putin for the SMO were 'demilitarization' of parts of Ukraine and 'denazification'. This was needed because of human rights violations in eastern Ukraine largely at the hands of ultra-nationalist Ukrainian units.

Violations that are well-known and documented by independent people like OSCE observer Benoit Paré in his book What I Saw in Ukraine, among others, but largely denied by the MSM. 

The mainstream western narrative has whitewashed all the human rights violations by Ukrainian ultra-nationalist forces who they falsely claim either do not exist or have no influence. And anyway, as they and Ambassador Holland laughably argue, Zelensky is Jewish so how could there be neo-Nazis operating in Ukraine? 

All this displays is a lamentable lack of knowledge of Ukraine's troubled history.

Something that, for example, the UK head of MI6 and lots of Ukrainian-Canadians know all about.

If these 'non-existent' ultra-nationalist forces were operating in the UK to the extent they do in Ukraine then Starmer would have a much bigger problem on his hands than a few 'right-wing thugs' protesting outside immigrant hotels.

International law was being violated and human rights abuses perpetrated, but by Ukraine (after 2014 and before the SMO) in the eastern regions and not Russia.

The sanctions are an illegal response to illegal activities. That's called 'doubling-down'.