2nd the traveljohns. Discovered them a few years ago and everyone in our group swears by them! Perfect for that need in the middle of the night when the cold air gets to your bladder and the last thing you want to do is struggle getting your shoes on and getting down to the portaloo / long drops with your torch
Also definitely go for a nice comfy inflatable mattress.
If you need your sleep, head for the quieter areas to camp such as Park and up near Pedestrian Gate A. However, these are further away from all the action and require an uphill climb back to your tent.
Take a lightweight collapsible camping chair with you that you can carry around with you. I went to a concert a few weeks ago at 30 weeks, and couldn't stay standing throughout all of it.
Comfy shoes are a must. Don't live in wellies as they will kill your feet - they're only useful if it's going to be a wet year (which it won't!).
Midwives are on site at the Medical Center up near the farmhouse if you do feel you need checking over. If you're at all worried it won't hurt to pop up there once you've set up camp so they know you're there, and they can register your tent location. Plus there's an NCT tent in the Kidz Field open from the Friday morning if you need their advice.
I would suggest travelling as lightly as you possibly can. Turn DH/DP into a pack horse and enter the site carrying the bare minimum. Alternatively take a very sturdy trolley for all your gear (pneumatic tires are highly recommended to cope with the rough terrain).
As for everyone drinking around you, I've found that sniffing alcohol is all I need these days when my friends are drinking. They all think it's hilarious! Seriously though, you will find it more tiring than if not preggers and might not get the same buzz as everyone else, but it will still be fantastic.
Oh, and if you're suffering from back ache I can highly recommend Deep Heat patches. You attach them to your clothing and they give off about 8 hours of warmth. Just like having a hot water bottle strapped to your lower back
You're going to have a great time . Really looking forward to it with PFB who will be just 4 weeks old if he arrives on due date [crazy hippy parent emoticon]!