Custard creams. A small sandwich biscuit with a vanilla filling
Chocolate caramel digestives. Digestives are about the closest thing to a "default biscuit" in the UK. The chocolate caramel ones fancy it up a bit
Some good shortbread. This is probably worth just making yourself; it's quite easy, and there are a lot of disappointing commercial shortbreads. Mary Berry's recipe is good. Don't bother with the almonds, and you can swap the demerara sugar with more caster sugar if you want
Where we live, there's an independent supermarket with blue badge parking. They do a certain biscuit there that you can't get it anywhere else. It's like a viscount with minty interior that my wife likes. It's a minty biscuit. It's a minty biscuit and it's got two halves, chocolate on the top. It's wrapped in silver foil. Right, she's very fond of them and it's got a sort of soft centre in the middle. A minty biscuit. And it's the only place you can get this minty biscuit. So sometimes in order to get it... It's got quite crunchy. It's a crunchy biscuit. A crunchy minty biscuit with chocolate. Well this biscuit, the crunchy minty biscuit that she likes, you can only get it from this particular shop. Well she likes this biscuit and sometimes I can't get there unless I use the blue badge to go and get it. It's a very nice biscuit. It's baked and it's a chocolate affair. It is delicious. Its very nice. It's a lovely one. I've eaten the biscuit, don't get me wrong. I've had one and they're jolly nice. They're mostly for my wife and the neighbour comes around, he's quite keen on them. He doesn't like the orange ones but he likes the minty ones and I've gone round and got that for them. The orange one isn't so nice it's the minty one really.
Empire biscuit - Scotland specific but you might find them south of the bordr.
Can be done badly (I'm looking at you Tesco) and come in sizes handy to "I'll never manage that but I'll give it a good try".
For reference: https://baynes.co.uk/product/empire-biscuits-2-pack/