Depends if you want;
Cheap
Intermediate
Good Quality / Easy Life
You can pick up a really cheap simple micro-cam with a few various brackets for about £30 from the cpc website. It is difficult to find though;
CNM LIFESTYLE|MD90-CAM|CAMERA, DIGITAL, MINI, BLK+2GB | CPC
Works OK but not brilliant quality.
As mentioned elsewhere there's the GoPro circa £200
But I just noticed (haven't seen or used) the
Nflightcam Cockpit HD GPS Video System on the transair website - looks sweet and has an adapter for recording by plugging into the intercom (but I bet it wont have a helicopter plug with it). Circa £300+
All of the above are better suited to mounting in the cockpit but the Oregon Scientific does actually come with a helmet mount - although everyone I know that has tried this series of cameras for mountain biking or track-days normally come back with pictures of the road or the sky - so might take some trial and error...
ATC9K HD Action Camera
Circa £1-200 depending on HD etc.
CC