Brown the mince in a large frying pan over a medium-high heat, breaking it up with a wooden spoon until no pink remains. Tip away any excess fat.
Add the onion and carrots and cook for 5 minutes until the onion has softened. Stir in the plain flour and the crumbled stock cube, then pour in the hot water and the Worcestershire sauce. Simmer for 15 minutes until the carrots are tender and the gravy has thickened. Season well and tip into a baking dish.
For the cobbler topping, rub the cold butter into the self-raising flour with a pinch of salt until it resembles coarse breadcrumbs. Stir in the cheddar, thyme and enough milk to bring it together into a soft, slightly sticky dough.
Spoon 6 rough rounds of the dough over the hot filling, leaving gaps between them so the gravy can bubble through.
Bake at 200C (180C fan) for 18 to 20 minutes until the scones are risen and golden and the filling is bubbling at the edges. Rest for 5 minutes before serving.