I think its just too big and too ambitious. It should be done at a race track, £5 entry or whatever + £5 for your your commemorative red shirt -or bring your own. get loads of tea-fish and chip and burger vans, beer tent, other motorcycle stands all of which will be paying a fee to the charity to be there. Get the air ambulance there who can charge for rides and once the ring of red is full swing they can get an aerial photo of whole ring and sell that too.
Start it all off with a 2 min silence and mark the end of the 2 min with a mas rev-up.
It would be a lot safer than the m25 and also would look much better and raise more money.
Oh and have it in the middle of the country somewhere.