The "Isotomurus palustris" group contain some of the commonest and most widespread springtails in the UK, and can be expected almost anywhere that is permanently wet. All Isotomurus possess trichobothria on the dorsal side of the abdomen (Fig. 4), though be aware that these have a habit of vanishing during vacuum collection, the vortex snapping off these delicate organs.. The mucro is also distinctive and has four teeth, the apical one being quite small (Fig. 5).
Their taxonomy has changed sufficiently that old records can only be put into a broad species group, and the 2007 AIDGAP key lump Isotomurus unifasciatus under the name "Isotomurus palustris". Undoubtedly the map for Isotomurus palustris contains many literature records that refer to Isotomurus unifasciatus, Isotomurus maculatus, and other Isotomurus before they were recognised as separate species and not simply colour varieties of Isotomurus palustris.
Isotomurus unifasciatus is probably widespread but records are hopelessly mixed with Isotomurus palustris. Steve Hopkin dismissed Isotomurus unifasciatus as a synonym of Isotomurus palustris and did not feature it in his 2007 AIDGAP key. Arne Fjellberg clearly separated these species in his key, based on enzyme evidence from Antonio Carapelli et al (2005), so I should have been aware of this before Antonio pointed out in 2011 that the "Isotomurus palustris" I photographed next to an Isotomurus plumosus was in fact the first UK record of Isotomurus unifasciatus! The photo is Figure 1 below.
The distinction between Isotomurus palustris and unifasciatus is that Isotomurus unifasciatus has a continuous dark line down its midline but is otherwise uniformly pale, while Isotomurus palustris has a slightly discontinuous dorsal line and mottled patterning on the sides of its abdomen.
The map below only shows records post-dating June 2011, but this colour form is common and familiar, so the species is probably as widespread as true Isotomurus palustris. Only extensive further sampling will resolve this, preferably accompanied by DNA sequencing.
Reference: Carapelli A. et al (2005) Assessing species boundaries and evolutionary relationships in a group of south-western European speces of Isotomurus (Collembola: Isotomidae) using allozyme data. Zoologica scripta 34, 71-79.