Re: To sadly turn this political
It's a matter of size and ability to resist.
If a Ukrainian ship (not that they have anything particularly sizable) had tried this, the Russians would have threatened, boarded, and impounded the vessel, subjecting the crew to some humiliating questioning and threats of 'espionage' before letting them back into Ukraine, although they would probably keep the ship.
The fact that it was a major vessel with significant offensive and defensive capability, from a navy that is well respected, which was testing an internationally agreed (with the exception of the Russians) trade route, they would be mad to do anything more than just posture.
If a Russian trawler obtains a UK fishing license, it would be free to fish in UK territorial waters. If they didn't, they could be quite legally prevented from fishing in these waters. We have fishery protection vessels to make sure this happens.