Re insurance cover - beware that there are limitations to its coverage. You will need to consult with the BCA Legal and Insurance Officer to get a fuller ruling. My guess is that winch meets are not covered since as I understand it, the GG clubs did a special presentation to obtain it.
Corporate manslaughter can only apply to a body corporate - so it is not relevant to an unincorporated club.
What is manslaughter is a mater based on the circumstances. The wiki reference seemed to give a moderate understanding - but it was not my area of specialism. It can only be used against individuals but again the circumstances will determine who from the club membership might get charged. And no one can offer any further guidance without a hell of a lot more detail of the circumstances. To give you some perspective, I have spent weeks collecting evidence and then days working out who best is to be charged. It ain't simple. I therefore doubt if you can provide sufficient detail for any one to give reasonably definitive advice (even if they were prepared to do so - given the potential for liability arising from that advice).
HSE may come and investigate a fatal accident or one where a serious injury has occurred for a variety of reasons which in part will depend upon the circumstances. The simplest summary is to expect it and be grateful if it does not arise.
The other point worth making is that in a civil case the claimant (the injured party) will potentially use as evidence any piece of advice regarding what is reasonable to have been done. So there is a hell of a lot of advice / legal requirements produced by the HSE which may well have an impact of the civil case even if the defendant is not charged under H&S law.
To understand the principal of a potential prosecution, one first has to identify the piece of law which it is alleged has been broken. Take as a simple example HASWA Sec 8 which states "No person shall ... interfere with ... anything provided in the interests of ... safety ... in pursuance of any of the relevant statutory provisions". (Don't worry about the ... for this example.) So the questions which I would have to prove are: that you were that person, that you did interfere with a piece of kit as opposed to just fiddled with it, which in turn means showing what it's safety function was and why your interference stopped it being available or why it was interference (what does that word mean? sorry a rhetorical question), that the piece of kit was provided in the interests of safety, which statutory provision required it, was the piece of kit still in a situation of for use at work (even though you might not be at work). And having just written this, I would almost certainly find I had missed a number of other points, like demonstrating the person who provided the lit was an employer.
Lastly, some debate was made about the reverse burden of proof. The relevant piece of law is Sec 40 of Health and Safety at Work etc Act 1974 as amended (see
http://www.opsi.gov.uk/acts/acts1974/pdf/ukpga_19740037_en.pdf for a copy of the original - I seem to have lost the location of a revised version) and states:
"In any proceedings for an offence under any of the relevant statutory provisions consisting of a failure to comply with a duty or requirement to do some thing so far as is practicable or so far as is reasonably practicable, or to use the best practicable means to do something, it shall be for the accused to prove (as the case may be) that it was not practicable or not reasonably practicable to do more than was in fact done to satisfy the duty or requirement, or that there was no better practicable means than was in fact used to satisfy the duty or requirement."
E&OE of course. So it applies more broadly than just situations described in ACoPs. Sorry for getting that wrong.
I am not sure that is much help, but what you are asking is in some respects like asking how long is a piece of string.