Typical portal drawing from official BR drawings, this one is a portal near Stafford
The real portal is 24ft 1inch overall height above railhead height, that's 96mm in 4mm scale
The laser-cut portals are 92mm clearance, so presumably that is the measurement from the foot of the leg to the underside of the portal bridge. By the time you add in the thickness of trackbed, sleeper and rail (about 6mm) that allows 86 mm clearance, allow 10mm for the depth of the portal bridge itself and that comes pretty damn close to the prototypical overall height for a portal. It would seem to me that the laser-cut portals are pretty much EXACTLY to scale and it is Bescotbeast that needs to read up on the subject matter.
As an aside I personally am not too keen on the look of the laser-cut portals, and I wonder how you would go about attaching the hanging gubbins for the wires themselves, N-Brass look much more convincing
(Second portal from camera is N-Brass)
Andi