I believe 18mph is related to the "design speed" of the facility: it's not a limit as such, it's advice that you may actually not be able to exceed that speed due to poor sightlines, sharp turns, etc etc.
If the DFT were to introduce a recommended speed for polite cycling on shared-use paths, I would hope it would be nearer 12 than 18 and I would like people to slow further while overtaking. If you want to do vehicular cycling, do it in places where vehicles generally are allowed: if you want to mix with the pedestrians, you play by pedestrian rules.