Virtually all gliders that fly cross-country will have radios, although this is not required by any current regulations. Their pilots will be operating on one of several dedicated gliding frequencies - mostly 130.100, 130.125 for cross country. When a glider pilot wants to enter cloud they should (must) change to the cloud flying frequency which is 130.400 and call their position and height (the latter at regular intervals).
Pilots will call FIS or ATC units when close to or wanting to transit certain classes of controlled airspace, obviously on the appropriate frequencies.
There are about 2,600 gliders in the UK. I would guess that c.50% to 70% of them could be airborne cross-country on a good thermal day, particularly weekends.
The performance of modern gliders is such that distances of 300 km in a flight is now normal, and 500km+ is done regularly. The UK record is c. 1,500 km in a day (in Scotland, in wave). Cross country cruising (gliding) speeds are generally between 60kts and 100kts, but occasionally up to 120kts, depending on the glider type / performance and weather / thermal conditions on the day.
To get some idea of the number of cross country flights in a year, go to
Ladder Home Here glider pilots log their cross country (or wave, height gain) flights. Not everyone by any means logs their flights on this website. Those that do have logged 2,900 flights so far this year, and 2008 has been one of the worst seasons for cross country gliding, weather-wise. Imagine a good year!
Gliders operate generally up to the base of (cumulus, mainly) cloud before setting off on a glide to the next thermal. On days when there is 'wave' the are likely to be operating above cloud and up to heights that can reach 25,000 feet (subject to airspace restrictions of course) in mountainous areas such as Scotland, Vale of York, Welsh border country, Eden Valley, Severn Valley. Most wave flights however tend to be from c. 4,000 ft to 15,000 ft.
Enough info?