Which helicopter mark is not associated with the Squirrel?

The designation HC2 is not associated with the Squirrel helicopter. The Squirrel, known for its versatility and agility, is primarily represented by the HT1 and HT2 marks within the UK military context, particularly used for training roles. The HT3 designation is also connected to the Squirrel, but HC, typically linked to helicopters used for combat or utility purposes, does not apply to the Squirrel helicopter models in the same way.

In summary, while the Squirrel helicopter has various designations reflecting its training roles, HC2 is not one of those, making it the correct answer.