Just had a look at the process and you need to write explaining why you want to visit to:
Swanwick Visits
Box 25
NATS Swanwick
Sopwith Way
Southampton
Hampshire
SO31 7AY
Can't get email address to display correctly on here (dunno why) but if you take out the space before and after the @ it's Swanwick.Visits @ nats.co.uk
BD