They do go down to that size, and indeed there is space to fit one. Think it was due to cost / practicality and what spare LED blinds GNE had in their stores, as a number of vehicles that gained LED blinds during refurbishment lost them when the vehicle was sold and withdrawn. Plus it be a headache for them to create a non-standard set of LED blind displays for a batch of vehicles. The ex Oxford Volvo B10BLEs had similar issues due to their blinds being wider IIRC.

Anyway some evidence of LEDS in the side blinds on Omnidekkas.
Lothian - https://www.flickr.com/photos/gjm-photogenic/9469714758
Preston - https://www.flickr.com/photos/gjm-photog...3606152189
Reading - https://www.flickr.com/photos/gjm-photog...9777806341

The Omnidekka wasn't consistent, it depended on the operator specifications. I'm guessing those above plus Brighton wanted the blinds in a seperate box above the first window to remove a blind spot for the driver whilst other companies weren't so bothered about it.