As for gliding the nearest to DSA is Gamston but they are moving from there.
Rob, you are correct - but they ain't moving far! I heard the new site for that particular club is perhaps 3-ish miles from Gamston. And why the smiley face? Are you supposing that the gliding club is somehow 'not worthy' of being in the area?
To answer your question in the simplest of layman's terms, Class D airspace is an defined area into which aircraft are not generally permitted without a clearance from the air traffic controllers who 'own' it. Once cleared to enter, traffic within it is obliged to follow the instructions issued by ATC.
Class D comes in two forms (normally used together) - Zones and Areas. Zones are from the surface up to a specified altitude. Areas are between two specified altitudes, which allows traffic to perhaps fly underneath the lower limit.
I suspect HOODED is nearest the truth on this. There are a number of
potential conflicts; nearby GA airfields, various gliding clubs, military ops, etc. Any of these could become potential objectors.
Time wil tell on this one; let's see what happens when any application goes to public consultation.