AFAIK because of legislation changes pilots must now have a Bronze to do a DI without supervision, so no more signing off in the log book. However we do teach doing a DI. Each type of glider has it's own gotchas - or at least each type I've done a DI on.
Sorry but that statement is factually incorrect. The BGA has taken that stance on the basis that a DI is a maintenance task but it is not supported by legislation - in fact the relevant EASA legal document states almost the opposite. To perform pilot/owner maintenance and to sign it off in the log book the pilot must hold a licence or equivalent (in the UK a Bronze), however Part M Appendix XIII states that:
Any task described in the aircraft flight manual as preparing the aircraft for flight (Example: assembling the glider wings or pre-flight), is considered to be a pilot task and is not considered a Pilot-owner maintenance task and therefore does not require a Certificate of Release to Service.
Since the DI for a glider is normally described in the FM then the legal requirement is that it be a pilot not a licensed / bronze pilot.