Ukraine on Thursday confirmed it wrecked a Russian submarine with British weapons, during a missile attack on the Black Sea port of Sevastopol in Russian-occupied Crimea.
The Russian cruise missile carrier — the Rostov-on-Don — was significantly damaged in the massive Ukrainian strike, as was Kremlin warship the Minsk.
A senior Ukrainian military official confirmed to POLITICO that Ukrainian pilots used the British cruise missile Storm Shadow for the attack.
What kind of red line? Sevastopol is on invaded ground.
Maybe they mean the supply of long range weapons by the UK
https://news.sky.com/story/russia-threatens-military-response-after-uk-gives-long-range-missiles-to-ukraine-12878786