Yawn, so the sub surfaces near the Russian spy vessel. The Russians wave cheerfully and carry on. The sub waves back and does nothing.
Brave Little Britain. Can't actually do anything as long as the Russians are in international waters UNLESS (I assume) they catch them red-handed in the act and therefore (I assume) breaking international laws ... Got to have good grounds for acting BEFORE you act. Russians surveying the cables but not damaging them is not actionable, I shouid think.