What are Storm Shadow missiles and why are they crucial for Ukraine?

Matthew Savill, director of military science at Rusi think tank, believes lifting restrictions would offer two main benefits to Ukraine. Firstly, it might "unlock" another system, the ATACMs. Secondly, it would pose a dilemma for Russia as to where to position those precious air defences, something he says could make it easier for Ukraine’s drones to get through. Ultimately though, says Savill, Storm Shadow is unlikely to turn the tide. Ukraine doesn't have many missiles, and the UK has very few left to give.