You are not charging anything for the RIB ride so it is not a commercial operation (no employees).
I would think as long as you are competent and your boat is fit for the planned trip, the winner is fully aware you are not a commercial operator there should be no issue.
You must inform YOUR insurance company and they may stipulate certain conditions - winner/s wear suitable clothing and life jackets, given a briefing before the trip etc.
I agree you should contact the charity and ask what help they can give, I know it may seem a little bit over the top but a little bit of planning can save a lot of trouble in the future should anything happen (civil claims).
As for risk assessments they are nothing to fear just think of what you intend doing on the trip and the passengers you will be carrying and ask yourself - what could go wrong and what would I need to do to keep everyone safe, do I have everything on board or do I need to add further items.
Remember you are familiar how your boat rides a wave, a passenger may not have a clue!
If you want more information you could contact: - Coastguard (MCA), RYA and local authority environmental health officer. Try the HSE website lots of information on there.
Hope this helps a bit.