if you dig a bit, you find that several various jamming trials have occured.
Here's advice warning of a series at STANFORD TRAINING AREA, EAST ANGLIA
http://www.ead.eurocontrol.int/eadba...2011-09-08.pdf
reports on trials carried out by the lighthouse authorities. Interesting reading
Maritime Jamming Trial Shows GPS Vulnerabilities | GPS World
Portreath in 2007
MoD boffins in Cornwall GPS-jamming trials ? The Register