There's no need to avoid overflying glider sites as a matter of course.
If it's marked as a winch site, overflying is dangerous if the site is operational unless you leave a confortable margin above the marked max winch height.
If no winch, treat the site like any other unlicensed airfield, which I guess means not blundering through the circuit.
Otherwise, wave as you pass - if I'm flying I'll wave back.
(BTW, gliders launching from Aboyne (near Aberdeen) have flown over pretty much the whole Scottish mainland and much of England too!)