Surely flying the Turboprop's at lower levels is just as dangerous as flying right up to the last minute the cloud is expected to hit particular areas. Although the cloud is currently above 20000ft, what's to say it will not descend? What does the turboprop do then? If you cannot see the cloud & something at 15000ft gets affected, not much airspace left to restart & recover. Would you want to carry passengers with that risk? That's why the airspace is being closed "early" just incase the cloud drift speed increases or the fragments actually fronting the cloud aren't being picked up on satellite.
European Airports are being affected as all flights planned to route through UK airspace after 11:00z are suspended and will have to re-route via France/Spain.