All national, and regional competitions in the UK, as well as some inter-club league competitions are NOTAM'd.
Any decent task setter worth their salt generally sets tasks away from areas of potential heavy traffic, and routes are normally set with good horizontal seperation from controlled airspace. Airspace infringements are another topic altogether!
It's very unlikely that in any field of say 40 glider that any more than the odd one or two, will take or risk a cloud climb, and thats only if the conditions are right. As I have said in earlier posts, it really is the exception - rather than the norm.
A system called FLARM has been introduced on the continent, which is aimed at providing a collision avoidance system for light GA and gliders.
It uses integrated I-band tranceivers and GPS to provide visual and audio feedback on aircraft on collision courses (360 degrees). Unfortunately, as I understand it, the frequencies used for FLARM, are allocated for other purposes in the UK. I for one would fully support such a scheme in the UK, given the relatively low cost.
http://www.flarm.com/index_en.html