Possibly Fern Smut moth larvae - link below on ferns generally with a section on pests says to destroy any plants or infestations you find. More common where there are lots of ferns. Google Fern Smut moth larvae (Psychoides filicivora) - Naturespot.org.uk has a single image of a larvae, there may be other images on other sites.

https://ebps.org.uk/ferns/growing/cultivation-ferns/

it is very likely to be this moth as bamboo says. It also is a good match for the images I have found.

it is another alien species probably as a result of importing ferns from Asia.

There are several photos of the adult which is a micro moth and its caterpillar on the dorsetnature site.

This is an excellent site for you to do some checking.

https://www.ukmoths.org.uk/species/psychoides-filicivora/

The adult moth is a day flier and about 1cm long and is a sooty black colour.
my daughters Field guide Caterpillars [ a mega tome that has all the native and commonest invasive species] doesn't include it so it hasn't been recorded officially enough to have it included in this guide.

You can send the image to iRecord which will then have people verify the identification or give you a correct one.