There are at least two definitions of severe icing in aviation.
First that used primarily in met forecasting; in the UK this is defined in
AIP Gen 3.5.
Second that which describes conditions encountered during flight and observed / reported by crew. This is defined in table 3.5.6.2 in the AIP, but note that there may be some differences between these reporting definitions and aircraft certification terminology (CS 25 / FAR 25 Appendix C), i.e. an aircraft may have an icing certification, but this does not imply that it can fly in all conditions, particularly not in severe icing.
Essentially severe icing relates to conditions which can overwhelm an aircraft’s anti / deicing system and thus the conditions must be exited immediately (diverts from the planned route). Thus this definition depends on the aircraft type and not so much on the type of icing or met forecast. Therefore in extremis, icing could bring an aircraft down, but it’s is more likely that individual systems would be rendered ineffective; e.g. engine, controls, instruments
BOAC, not so much a judgement, but a comparison; the rate of ice accretion is faster than the rate of removal by the aircraft system – airframe, engine, windshield, probes, etc; possibly as simple as ice remaining on a protected part of the airframe.
An ATIS broadcast can be of either form; the more likely is a warning issued by met, the alternative of a report from a crew (PIREP) would ideally specify the type of aircraft encountering the condition.