Hi Cleo! Welcome!
Okay, my input as an American living in Britain.
Cookies are large (2-6 inches diameter), soft, chewy and yummy. Good with a glass of milk.
Note: the UK DOES offer cookies and they are usually labelled 'American cookies' and are almost always chocolate chip or white chocolate. I had one today. Here is the label.
UK biscuits are hard, dense, usually small(3 inches or smaller so they can fit in a standard mug for dipping), can be eaten alone but are better dipped in tea in order to not crack your teeth on them. They taste good but not as good as cookies.
US biscuits are large (3-4 inches in diameter) soft, buttery and served with savoury 'breakfast' gravy made with flour, sausages and milk, usually white/cream in colour (as opposed to British sunday dinner gravy which is from beef drippings, beef stock cubes for the brown colour, flour, etc). They are soft and fluffy if cooked correctly and can be eaten alone or with a carvery type meal as well.
UK scones are smaller in general with average sizes at shops being 2.5 inches in diameter and I've had scones larger but it's rare. They are usually a standard size. Served with clotted cream and jam, or regular whipped cream and jam. Some people just have them with jam. Usually accompanied by a cup of tea. UK scones are never eaten as a savoury dish. There are two flavour choices usually and that's plain or sultana (a type of raisin from a different grape than American raisins).
US scones are similar to UK scones in flavour, but they are a bit dry, usually sprinkled with sugar and usually triangle shaped. Common flavour choices are usually plain, blueberry, or other berry or fruit.
End note:
US cookies are superior to UK biscuits.
UK scones are superior to US scones.
There is no equivalent to a US biscuit in the UK. But the type of breaded item served at Sunday dinner or a roast carvery are Yorkshire Puddings which are little baked fluffs of goodness made from egg, flour and water (or milk). Google them. Make them. Eat them.
Sorry Cleo for that huge thread hijack but I wanted to get that out there.
Again, welcome. I hope you enjoy your time here!