I'm not privy to any inside information but I understand from what I read that the feature is not present on the latest satellites. According to an official US government statement at gps.gov
The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without disrupting or degrading civilian uses outside the theater of operations
Localized jamming and spoofing by US, foreign and non-state actors is always possible, as has been recently reported in the online technical media.
There is a website which for us users might be useful: it's designed for pilots to check forecast gps disruptions prior to flight, but I imagine it might be of interest to ground-based gps users too.
http://www.raimprediction.net/I expect that when the UK military plan gps jamming activities they coordinate with the CAA and relevant notams are issued to pilots well in advance. I would be surprised if was anything other than announced months ahead,