Glentress is a trail centre and therefore it has similar trail conditions to those found in Welsh trail centres.
Suspension forks serve to provide comfort and control over rough ground and trail obstacles.
Due to the nature of the trails one would expect to encounter at Glentress, having experienced the Welsh trail centres, a large volume tyre would suffice in place of suspension forks on a rigid bike.
Trail centres are known to lack loose technical climbs and steep ascents and are therefore inherently suitable for singlespeeds, the use of which avoids the problems and complexity of dérailleur gears.
With a tyre of suitable grip a singlespeed would be more than adequate for most trail centres.
For these reasons and drawing on previous knowledge I would recommend the use of 2.35" Schwalbe Fat Albert tyres for a rigid SS at glentress.