[Flyin'Dutch']

gliding sites, one can, I believe, safely assume they are inactive on blowy, wet autumnal days.

Wet yes, blowy not necessarily. Always trainee to train and pilots keeping current.
And if a ridge or wave lift are accessible, windy day might see a lot of activity.

What would be the operating limit on wind speed out of interest?

Until you are blown over there is no need to cease gliding operations.

[xtophe]

What would be the operating limit on wind speed out of interest?

Obviously a lot of parameters: gusts, turbulence, direction, experience, ...
I'd say between 25 and 30 knots.