I see what you mean now. The NATS website state that they cover 15 of the UKs major airports of which Prestwick is not listed, however, if you visit the NATS careers website and roll over the 'working environment' tab it is listed in there as:
Prestwick And Oceanic (Are Control Centre)
Glasgow (Control Tower)
http://natscareers.co.uk/indexfs.asp?id=200676
It could just be me however because i thought Glasgow only had one airport which was Glasgow Prestwick. If that is the case then i must apologise.