Isotoma caerulea would have been identified as Isotoma viridis until the final version of the UK AIDGAP key came out, so is barely recorded at all despite being probably moderately common. Isotoma viridis is now split into four species, Isotoma viridis, Isotoma anglicana, Isotoma caerulea and Isotoma riparia. (All species have three teeth on the mucro). Isotoma riparia only occurs in very wet habitats and has a distinctive stripe down the middle of the body - much like Isotomurus palustris, but without trichobothria. Isotoma viridis Isotoma anglicana and Isotoma caerulea can only be reliably separated by examining the manubrial 'teeth' and the chaetotaxy of the dens. Isotoma viridis has a single pair of 'teeth' on the thickened apical edge of the manubrium (Fig. 1) whereas Isotoma anglicana and Isotoma caerulea have two pairs of manubrial teeth. The latter two are separated by the basal dorsal setae on the dens (3= anglicana, 2 = caerulea). The literature records for these three species must be regarded as hopelessly mixed up.
Frans Jansenns tentatively identifies Isotoma caurulea from its blue irridescence, as here, (caurulea means blue), but this seems a bold step based on flash photos!