Its a good question. First, after Crimea and Salisbury, the UK government along with the US and other allies/partners has taken action short of getting into an escalating crisis: sanctions, expulsion of diplomats, etc. The reality is that there are only limited options... https://twitter.com/Charlie533080/status/1301283662483386368

...short of taking on very high risks and destabilisation. We also still have important interests that depend on our capacity to bargain with Russia effectively: nuclear arms control, the need to balance a more powerful and oppressive China, and (if you think it matters) Syria.

I'm afraid also that the UK, like all major powers, flouts international rules when it believes it serves its interests/greater good. Which can be defended. International life is conflicted, tragic & difficult - and not reducible to applying codes of conduct equally & everywhere.

That doesn't mean equivalence - Boris isn't arranging for Starmer to be poisoned, or barrel bombing rebels in Syria. But on military strikes w/o UNSC resolutions, Diego Garcia, or long-forgotten rendition during GWOT, we too from time to time violate rules to pursue security.

One final issue is that the "rules" are laid down via institutions like the UN, where Russia is a permanent UNSC member with a veto. So the rules themselves do not stand above politics, and themselves can be in conflict. Its a murky world.