FWIW, the military have an active policy of avoiding gliding sites, although all the FW assets can easily overfly by a huge margin.
I've been at the Long Mynd in Shropshire when a gaggle of Hawks came over at 300ft and pulled bl##dy hard when they realised they were over a gliding site, a quiet word sufficed as they are normally quite good at avoiding the site.
One other thing to consider about gliders is that if the thermal is still going at cloudbase, some gliders will enter cloud until the thermal peters out and then pop out of the cloud.
If the military see fit to avoid gliding sites, it's just not worth introducing the extra element of risk. BTW Long Mynd gliders generally have radio which is used to good effect with traffic out of Welshpool which is relatively close.