Comparative image of girdle tufts on A. fascicularis. Several specimens from Adriatic in Croatia. © J. Prkic.
Both A. crinita and A. fascicularis have a range of colours possible on their tufts; the colours are not diagnostic for differentiating the two species.
Except for A. discrepans which has features integrading with A. crinita, and is regarded by Jones & Baxter and others as indistinguishable, the only other British species with eighteen tufts of bristles on the girdle is A. fascicularis (above).
However, A. discrepans is a recognised species on WoRMS; their image is at www.marinespecies.org/aphia.php?p=image&pic=70144

KEY ID FEATURES and similar species at: 01 Acanthochitona crinita