cyclehunter
Folkestone to Calais is mapped with route=shuttle_train tunnel=yes and bicycle=yes
https://www.openstreetmap.org/way/209213884#map=11/51.0448/1.5947
There is no highway so I think it is clear that you cannot cycle the tunnel.
Here we have a highway and you can cross with a bicycle, but not directly.
I propose to mark the tunnel as bicycle=no and add a way with route=shuttle_bus tunnel=yes and bicycle=yes.
This way you cannot cycle in the tunnel but have a shuttle service.