Along with five other British webcaps, C. britannicus was selected by Kew Gardens as a highlight of taxa described by the organisation's staff and affiliates in 2020.

Cortinarius britannicus was described in a 2020 research note in the journal Fungal Diversity by Kare Liimatainen and Tuula Niskanen.

In a year-end round-up, Kew scientists selected ten highlights, one of which was six newly described British Cortinarius species: C. britannicus described from Caithness; C. scoticus and C. aurae from the Black Wood of Rannoch; C. subsaniosus from Cumbria; C. ainsworthii from Brighton; and C. heatherae from Heathrow Airport.

[2][3][4] In a press release, Kew identified Cortinarius species as "ecologically important in supporting the growth of plants, particularly trees such as oak, beech, birch and pine" and playing "a key role in the carbon cycling of woodlands and providing nitrogen to trees".

The spores are moderately to strongly dextrinoid, meaning that they stain reddish to reddish-brown when Melzer's reagent or Lugol's solution is applied.

The pileipellis surface is pale, consisting of more or less parallel hyphae that are 5 to 13 μm wide with zebra-like incrustations.

C. britannicus can be distinguished from other members of the subgenus by its combination of small-sized, purplish-coloured mushrooms and almond-shaped, medium-sized spores.