I haven't read through ALL the replies to this post because there are so many pages!! So apologies if this has already been said...
I have recently taken my Grade 7 after also having a huge gap since my Grade 6 (15+ years) and was similarly terrified of performing in front of an examiner!
However, I was able to arrange for a practice session on the actual piano I would be taking the exam on prior to the exam. This helped me enormously since I could check out he environment, get used to the feel of the exam piano. All I did was phone up the AB and ask for the phone number of the centre. I was able to book an hours practice, it was fabulous (not least because it was a beautiful grand piano!). I came out buzzing from the practise!
The only thing that put me off in the exam was when I turned a page I just caught the examiner in of the corner of my eye and it flustered me for a page!! So, if you have any page turns, make sure to focus focus focus!!!!
Most of all, good luck! I'm preparing for my grade 8 now so will be doing it all again soon!