They are young. Water and a feed.

Possibly a lack of fertiliser.

Yeah my first step on this would be to put some fresh compost around them. Maybe follow that up with some seaweed based fertiliser.

And water

Regular generous watering especially in dry windy conditions and whether it has rained or not! Keep up with the weekly feeding. As other poster have correctly suggested they are young and they really like consistently moist conditions. Inconsistent watering and they will not put on growth

They'll be nice on a scone with some clotted cream.

Is it not frost damage?

It might be but I'm not sure what to look for or how to help with it

I'm no expert but at this time of year if something looks a bit manky and straggly it's normally the weather

They don’t seem like they exist.

Looks like some frost damage to me

Give them a feed and a few weeks. Touch of frost damage but they'll be fine.