AIBU to think a Russian ship firing in home waters matters?

No we shouldn't, because that isn't what happened.

What I've been hearing/reading from maritime/ex-martime professionals ties in with a few of the above comments.

The Russian vessel was in international waters (and possibly for reasons unknown not in a position to manoeuvre, so sail v. Steam rules of priority may have been irrelevant)

Rumour has it the yacht was on a collision course, didn't respond to radio calls or warning by siren/horn - the point made by @ItsmeMargo ,.so louder noise was required...

Interestingly a few ex-Navy types reckon in this day and age of increased threat levels it's likely a yacht drifting towards a RN or USN vessel might have got a warning delivered in a similar manner.

It’s a bit alarming but the Russian ship was in international waters. The couple from the yacht were on BBC radio Sussex this morning and said the Russians didn’t fire at them but just to warn them. They dispute that the Russians are saying they sounded the horn etc to alert the yacht to a potential collision because the couple said that absolutely wasn’t about to happen.
Apparently there was another incident in the Channel recently with a Russian ship but I didn’t catch what happened there.

It wasn't in home waters. It was international waters about 23 miles from the Isle of Wight

What do you want? War??? That’s not going to end well.

Perhaps I’m being naive, but I don’t think it’s such a big deal. Russian ship sounded on the horn, which the sailboat admitted was to make sure they had seen them, sailboat then altered their course by only 2°, which is a very little amount – so the Russian ship sounded the horn again. The Russian ship then fired into the air above their own vessel. They did not fire at the sailboat. Apparently it was foggy, the Channel is a very busy waterway. The Russian vessel was there legally.