You will love the Nationals, a really humbling experience. Make sure you have good boots for walking in, you may have to make a lot of trips from stables to arenas and back with forgotten items, score checking, horse changes etc etc.
As regards some of the rather detrimental comments on previous posts, yes, walking round the arena can be boring, but you have to remember that for the riders on board it is still pretty exciting. Many are worried about "change" so things are kept the same on purpose. In our group we watched the ponies and horses very closely for signs of unhappiness or discontent and new homes were found when neccessary. Ponies/horses were/are hacked out on the moor or round the lanes at the week end, some did PC, HT and summer shows. We had a piebald cob who was a prolific show cob winner. Health and Safety and Safe Guarding are huge issues, but they really have to be given one is dealing with Horses (can be dangerous) and people with disabilities (can have erratic behaviour) so one is in a very "controlled" environment. Some of the older generation who have time to come and be volunteers have years of horse and people experience behind them and are invaluable.
Younger volunteers are hard to find and keep because of work/family needs of their own.
I did 20 years as a group instructor, I hope you will find it as fulfilling as I did.
We had Regionals this week and it was incredibly moving and inspirational to watch our riders competing. Our horses, in the main, loved being somewhere new, particularly our big Comtois who was very jolly. Having the Countryside Challenge as a competition class is such a good idea too - dressage movements without riders thinking that it's dressage, very clever!