Halfords claim "
Max range up to 40 miles". They do not claim it is something you'll realistically achieve every single ride, so I see no reason to be annoyed. In warm weather, on flat terrain with light rider and intellogemt use of the modes it is doable.
As an owner of the same model myself I can confirm that 28 is not an unrealistic mileage - I can do more than 28, but I can drain it in as little as 22.
However, if youre genuinely averaging 16 to 18 you should hardly be using any juice. You're either misrepresenting the
average speed (I'm fighting to to that as an average on my carbon roadie, never mind on my 18.5kg Subway E, so I do question if that really is correct) or you really do have a problem.
Also be aware that cold weather can hit ebike batteries hard. My own experience of the Subway E is a 20% or so drop in range for a drop in temperature from 12°C to 2°C.
Use your mobile phone and the Strava app to record your ride independently and get an authoritative average speed, then look again. However, your mileage is within the range that Subway E riders report, and that I experience myself, so I suspect there's little wrong with it - they're great bikes, but 317Wh is one of the smaller ebike batteries out there.