Vortex wake from a helicopter is the same vortex wake from a fixed wing as far as ATC is concerned.
Vortex wake between departing aircraft
must be applied by ATC, no excuses anywhere along the line.
I would estimate that a Merlin is a "small" wake vortex category aircraft, and therefore the ATC unit should have applied a minimum of 2 minutes between their departure and yours.
However, as it was a military ATC unit, it's not entirely suprising that they didn't have any appreciation of the consequences...
