I've recently noticed that my DVD copy of The Snowman is a PG. To me, this seemed absurd, simply because I couldn't think of a good enough reason for it not to be a U rating. Anyway after a bit of internet digging (I ended up on the BBFC website) I've discovered that the film itself IS rated U. The DVD's PG rating is due to a documentary found within the extra features.
The documentary is called "Snow Business - The Story Of The Snowman". Out of curiosity I watched the documentary and at about 14 minutes in, a lady is talking about merchandised products and briefly refers to a product unsuitable for children.
This must be the reason for the higher rating as a DVD has to have the highest rating of all of the content. For example if you bought a box set of three films where two of them were 12's and the other was a 15, the box set would be a 15.
I hope this information helps anyone who, like me, was baffled regarding The Snowman's PG rating.