In this instance, as the 63 only operates during daylight hours I'd argue that a clearly printed route number and clearly printed destination on boards/paper displayed on the front and side of the vehicle would be within the spirit of the Regulations if not totally compliant - an effort has been made.I guess that's the biggest issue with paper destinations! No easy answer either - there are bound to be regulations on how bright, controlling light spill etc.
A hand-scribbled number partly obscured by the wiper arm less so.

