A couple of years ago the CAA's AIWG (Airspace Infringement Working
Group) organised a coordinated 'visitATC' event, where ATC
units, especially those with radar rooms, were encouraged to host
visits by local pilots. These visits are often available anyway locally
if your club or school requests it direct from your nearby ATC, but this event was to put dates out on
offer. It was over-subscribed. It's happening again - go to http://www.airspacesafety.com
and look at the news (bottom left). There are already some ATC units and dates there for the end of May,
and more to come. Numbers on any one visit
are bound to be limited by practical problems of large groups, so these visits are for qualified pilots only, but clubs/schools can ask for a visit at other times
for students as a normal part of their training. The difference here is that ATC
units are going out advertising dates, where-as they will usually be ok
about being asked for them during the year for small groups and students.