It does vary a lot based on how much effort you put in yourself.
SOme cheaper ebikes (like the subway - I know it is >£1000 but.....) can tend to see that the pedals are moving and take that as enough evidence to deliver power. More expensive ebikes have better systems and look at the torque (i.e. how much pressure you put on the pedals) and use that to add somem# power - so if you press less hard the motor helps you less. The cheaper ones are not so sensitive so if you spin the pedals at a certain speed then you get full power without any real effort
On one of those bikes it can be quite difficult to stop the bike delivering power when you don't need it!!
I tend to find that this can sometimes only be stopped by riding close to - and edgeing over at times - the motor cutoff speed. This should be 15.5 mph but tends to vary a bit - having a speedometer can help here.
Also - and more importantly - if the bike has been in storage for some time before you took delivery then the battery may not be in great shape. The batteries do not like being left unused to long periods of time - so there may be a problem.
On that note I would contact
Halfords and tell then about the very low range ASAP. If only to register that there might be a problem so in the future they can't say the bike is too old now or that you have not stored the battery properly after a year or so.
Remember that Halfords stores vary a lot - some appear to be great - some not so much. And some have a great technician - but if that specific person is not in then the rest are not as good. So make sure the store manager knows about the possible problem.
If you get lucky they may replace the battery???
Anyway - register the problems and see what they say - in the meantime read the user guide and make sure that you follow the battery care instruction fully - it may help.
WHatever happens - keep us informed - it may help others to know how you get on
Good luck - and enjoy riding!!