Jam is a bit misleading. The US DOD run the show. If they wished they could simply re-enable SA with whatever parameters they wish to interrfere with accuracy even to the extent of limiting the accuracy to certain altitudes.
Those tests have been conducted here without affecting vehicles or ships.
You don't know the MOD, if they say they are going to 'JAM' the signal, trust me that is exactly what they are going to do.
How they do it, well I would love to know but you can bet they are
Jamming Exercises as stated in the AIC referenced in this thread and will be capable of messing up vehicle based GPS units within the stated range. It might occur to you that as the US DOD run the show the MOD might just like to make sure they can locally block the signal if the US DOD don't want to play ball one day.