Hi Sam, I don't think we've met but it seems like a massive shame that we haven't. I'm an event manager but, I'm also about to start my final year studying physics at Salford. I'm working on a few projects that involve various bits of complex electronics and embedded processing so I'm very interested in your event.
Right,
The first thing you need is the purple book:
http://www.hse.gov.uk/pubns/priced/hsg195.pdf (online version)
http://www.amazon.co.uk/Event-Safety-Gui...5094&sr=8-1 (print version)
This is the government guidelines on event safety. Essentially if you use this as a guide (obviously a lot of it won't directly apply to you but you can infer from it) you should pass all H&S requirements for events in the UK. There have been rumours for years that a new, more generic, guide is being written but it has never materialised so we all still refer to this one.
In the middle of the book somewhere (although I'd advise skimming all of it) is a way to calculate toilets / medical provision. It's well worth following it through even if you decide not to use the exact numbers it gives you.
Your biggest problem is going to be net access. Getting high speed net access for large numbers of people in rural locations is absolutely determined by the local cable networks. I think you're probably looking at a dedicated 'leased line' from a provider like Metronet
http://www.metronet-uk.com/Solutions/ManagedTemporaryBandwidthandCircuits.aspx (depending on the area you may be able to lease direct from BT but I'm not too sure.
Most require annual contracts but a few (like the one above) will do short term deals. They are not cheap though. You could be looking at 100's to 1000's (probably £1000+) depending on the required bandwidth. Given the numbers you're talking about this should only equate to a few pounds each though.
I'd start contacting leased line suppliers about this as soon as you can. Maybe give them an idea of what you are planning so that when you have a potential site they can quickly give you a quote.
Power is easy. If the site doesn't have enough you can hire a generator (or two?). Generators / cabling / other electrical stuff, is easy enough to hire but needs to be accurately planned and include some redundancy.
If you have any specific questions feel free to post them here or drop me a message on facebook. My profile is /seyemon