Russia will not attack all British ships in the Black Sea, possibly the ones delivering weapons, but even in this case there is a more "peaceful" solution, like naval blockade similar to the one made by USA during Cuba crisis.
Also now there is an official statement by Russia about such weapon shipments.
Now about "wrong hands", there is more then 1 type of such "wrong hands":
Enemies of the West
Pro russian groups don´t need to invade Kiev or any other territory held by current Ukrainian government to get western weapons into their hands. There is some easier way to do that, as the Ukrainian conscripts are not really high motivated. Most of that Ukrainian conscripts were forced to join the army which is the main reason for their relatively low motivation. It´s not such big problem to get the western weapons from that low motivated conscripts when they surrender or even run over, some of them are ready to sell that weapons for some good money.
Russia already claims to have captured some western weapons in Ukraine.
Also there is a video claiming to show Ukrainian soldiers selling weapons to their Donbass counterparts. Unfortunately there is no english subtitles.
Evidence of resale of weapons received from NATO by Ukraine is published
Friends of the West
This is a bit problematic definition of a friend as an enemy of the enemy. So this "friends" are nationalists and even islamists. I know it sounds like some kind of Russian propaganda, specially about islamists, but there are western sources proving that. Also not all that nationalists are automatically nazis. This both groups are very high motivated and experienced in what their doing even before 2014. They will not easily surrender to any Russian or pro russian troops and definitely never collaborate with them. The risk western weapons would go from that groups into Russian hands is about 0. The Ukrainian nationalists see Russian as their main enemy but they are not really friends to Poland and some other neighboring countries to the west of Ukraine. Among the nationalists there are not just Ukrainian ones, nationalists from some western countries are fighting among that nationalists groups, they could take the weapons later to their home countries. Now about islamists, most of them are Crimea Tatars or Chechens. These groups are also very hostile to Russia, but sometimes to the West, as we saw in Boston or France.
This is more or les the risks from supplying Ukraine with weapons. To decide how high the risks are is very difficult and speculative.
Here an article from non Russian source about islamists fighting for Ukraine. And some more references about islamists:
And here about nationalists. Some more references about nationalists in Ukraine. Also here a trustable sorce, that definitely can't be called Russian propaganda.
Some references about Ukrainian soldiers surrendering:
Also there is a statement from Russia about such arms shipments here.
CNN: U.S. admits their weapons could end up with Ukrainian terrorists
There is even a conection between Christchurch shooter and Ukrains far right groups like Azov as can be seen here and here. So an other "Christchurch shooter" with some of that weapons is not a good idea.