It is not a flea, nor as small as a flea (about 5 mm long). It can jump, and lives among moss but it is most easily seen when it walks over snow. It is a type of scorpion-fly, harmless despite the intimidating name. The adult snow flea, which lacks wings, comes out in the coldest months between October and April.
Snow flea (Boreus hyemalis) on moss
© Roger Key
In the picture, note that the head has a very long downward projecting beak (which has biting mouthparts at the end): this is characteristic of scorpion-flies. The dark metallic reflections on the body are feature of the snow flea. This is a female, which has a projecting ovipositor (at the hind end) which has an orange tinge. In some respects it look like a small bush-cricket, but the antennae are not nearly as long.
Scorpion-flies comprise the Mecoptera, considered the most ancient Order of insects with a complete life history of egg, larva, pupa and adult. In Britain there are three species with wings (in the genus Panorpa), two of them being common in the summer. Both larvae and adults are predatory on other insects.
The snow flea, Boreus hyemalis, is adult in the winter when temperature can be too cold to fly so they crawl and jump rather than need wings (the male wings are modified into curved spines used in mating). Exactly how they manage to jump up to 5 cm without muscular hind legs is not obvious. The build is dumpy, unlike the narrow body of a true scorpion-fly.
The larva lives in a channel in the soil just underneath mosses and pupates at the end of the burrow. It is predatory, like the adult, and active during late spring and the summer.
The snow flea avoids the mild south-west of Britain, preferring districts with a harsher winter (from Kent to Scotland). It seems to prefer sandy soils, as associated with heathy districts. However, it is rarely seen, mainly because people are not searching moss in mid winter and not bothering to see what comes out to walk over snow. Now there's a challenge for a keen naturalist!