The BBC Report map seems to suggest that the helicopter came from Portland, but it did (as the text suggests) come from Lee on Solent, at the top right of the map, underneath the UK overview. Thats about 100nm's, and I guess answers the question raised above over who's responsibility it is to cover the Channel Islands.
I'm sure the AB139 would make the distance OK, and probably about 15 mins quicker than the S-61, but I can't see it carrying 12 casualties. Can't remember the record for the back of a 61, but I know they got in a hell of a lot once !
The channel is still one of the busiest shipping lanes in the world, with several cargo ships having come together in the past few years. It is also one of the busiest passenger ferry routes (although that may be a pre Channel Tunnel statistic) so hopefully this incident will make HMCG have a quick think about the type of aircraft they want to use on the south coast.