As you no doubt know, the Global Positioning System was originally deployed from the USA for military purposes, and civilian uses have developed since then.

Occasionally, the British Military conduct tests on systems which jam the low power GPS signals in a local area, thus stopping receivers working for the duration of the tests.

Ofcom has launched an email update notification service to give advanced notification of these tests.
You can sign up for these updates by registering your email address at:: http://www.ofcom.org.uk/static/subscribe/select_list.htm