From the RSGB website:
The MOD will be conducting GPS jamming exercises in Mid Wales between 7th & 11th September, & between 14th & 18th September.
These excercises will take place between 9AM & 5PM within 5Km of 52 degrees 0.881 minutes North, 3 degrees 38.518 minutes West. This is the Sennybridge training area.
I’m not sure if this will affect any SOTA summits but it may be worth bearing in mind if you are passing through the area.