So if you're funded entirely by voluntary donations it's OK to kill someone? Really?
The same applies to loads of charities in the UK, including many of the bigger ones, and I can't see how it's acceptable for them to ignore safety regulations just because of their funding source.
The situation is clear with employers, they are responsible for the safety of their workers -- including any unpaid interns/volunteers who are working for them, so being unpaid isn't an excuse to be unsafe.
So the same would apply to paid employees of a charity (obviously), they're doing a job. IANAL but I assume unpaid volunteers doing work for a charity are in the same legal position as unpaid interns/volunteers working for any other company, the charity is still responsible for making sure their working conditions are safe.
Safety at work (or play) is also not something you can agree to sign away, for obvious reasons (e.g. rogue employers) -- even if you're doing something dangerous and sign an acknowledgement of this, there still have to be reasonable safety precautions taken, and if this is not done or they fail the employer is liable. You can agree to clean windows on a skyscraper dangling from a rope, but if the rope snaps and there's no safety line or similar as a backup the employer will still be liable.
An individual worker -- for example, whoever did whatever lead to the accident -- may also be found at fault, but only usually if they ignored safety precautions which were put in place. If there were no precautions or policy about them, the employer/charity is at fault. It's why people are made to sit through those boring safety briefings on how to do stuff and sign off that they've done the training and understood it...
Building sites -- regardless of whether owned by Wimpey, or CART, or the Wilts and Berks Canal Trust -- all have the legal requirements to take accepted and reasonable safety precautions to protect their workers. If they don't and tragedy strikes and they're found at fault -- as here -- then they'll be fined heavily. The Trust should have known this, and made sure the site was safe...