HTB - that road surface+4.5m/+4.8m rule applies to an open road to take account of traffic using the road. Waddo stops it's traffic so that +4.5/+4.8m rule doesnt apply and they can use the road height as it's reference.
The easiest way to visualise the approach surface is to look at the approach lights. The approach lights will usually be set to 'sit' on the bottom of the approach surface. You can see from that video that the spectators are standing and are taller than the approach lights - thus they are infringing the approach surface.
With Railway crossings there are Byelaws of Tresspass on Railway premises which they can use but with roads the offences relate to passing the red light. If the pedestrians are already past the red lights when they start flashing then they don't commit an offence.
However, they are technically committing offences of endangering aircraft safety by infringing the approach surface meaning that Police can use force (i.e. move them on) to prevent the offence.
Alternatively, as they are on the highway (legally defined as including verges and footpaths) they are required to obey the directions of a constable to proceed in a particular direction.