"Why do they not call the most appropriate ATC unit for a meaningful service"
...Perhaps because it isn't always easy to establish which is the most appropriate ATC unit ? As an example, if climbing above 3000' in cloud near Bedford, should I called Brize Radar, Benson Zone, Cranfield, London FIS, Luton ? (apols if this is a poor example, I don't have a half-mil map to hand to use a valid case but it hopefully makes the point). The most likely traffic to be in a cloud in Open FIR is going to be another glider (or several) on 130.4. Therefore it makes sense for a common frequency to be used (although it would be nice if some more of the radio spectrum could be allocated - say a North and South frequency, using a half-mil map to differentiate which one to use).
A Position report is given before entering cloud...e.g. "Glider 123 entering cloud 3 NM SW of Ely, 3500'". Subsequent position reports are primarily to give height "Glider 123 is remaining in cloud, climbing through 5000', last known position 3NM SW of Ely". As there's no point in cloud climbing in 30kt winds, then a guestimate is generally sufficient. GPS can also be used as an additional aid.
Denhamflyer - David's thorough response describes well the areas that gliders operate in relation to their home airfield - we often operate hundreds of miles away from base. If we're flying on a cross-country day (e.g. cloudbase of 2000'+, nice Cumulus growing, 5/8 cover) then you'll probably discover that there aern't many gliders near the home airfield as we're all making the best use of the conditions to go fly for hundreds of miles elsewhere!