Generally, the airframe will only be below freezing if the air temperature is below freezing. And if the air temperature is below freezing, you'll get snow, not sleet (RASN, in the UK sense of "sleet").
I wish I had kept the METARS but across northern Scotland last week, there was the scenario of SN and +ve air being reported simultaneously.
It is interesting that there are two very experienced IFR helicopter pilots who question the wisdom of discarding snow as a potential form of icing. I agree that the formal theory about snow means it won't stick. But in this country whilst we get SN forecast there is a risk it is not really SN, simply because it doesn't really get properly cold in the UK (unless you live in Braemar).
As a helicopter pilot myself with a similar number of hours to PACE, all of which are below 5000' and 150kts I have seen what looks like snow stick. In the context of this forum and the original post, I would say that SN on a UK TAF/METAR doesn't necessarily mean icing, but nor does it mean icing won't happen.
Whilst it may seem that helicopters are snow magnets, we probably have more in common with the Private fleet than it would first seem - similar speeds and operating altitudes.