Nimbus,
As your friendly ATCO said, MOST gliders show up on radar, but from experience they might NOT. In any event, these days the UK doesn't have full LARS coverage. For example, Coventry can't always provide a radar service and some military radar units stand down during holiday periods.
Even where radar coverage IS available, a primary contact could be anything from ground clutter to airways traffic. Some radar units don't have secondary radar, Coventry being one example.
If an aircraft painting only a primary contact on radar isn't in contact with ATC (or not transponding mode C) there is no way of them knowing its altitude, whether it is IMC or VMC below and certainly no certain way of knowing if the contact is a glider.
This is a flight safety issue equally affecting all of us. It would be better if we all worked together in such cases, not adopt an unhelpful "us and them" attitude. If an accident were to occur in these circumstances, legislation might well be used to prevent a recurrence........ GA already carries radios, transponders and often TCAS and speaks to ATC, so where do you think any legislation would be aimed?
Or do you have an alternative solution to the concept of a glider pilot simply making a radio call to ATC?
I note your comments about glider pilots being more in tune with their aircraft. Can you also sense a 737 coming along behind you at 250 kts in the same cloud?