Your speed is outside the lowest estimate and as has been stated your noise margin is higher than the optimum of 6. This can be an indicator of a fault on your line.
You should contact BT and request that an engineer be sent out to investigate the problem. If it has been like that since the initial install make sure that Customer Services are aware of that.
As regard difference between Infinity 1 and 2. Apart from the speed if your line can support it, there is no £30 activation fee and you get 50Gb of BTCloud storage with Infinity 2. Obviously not worth the difference if you don't get Infinity 2 speeds but I suppose some people may want it.
the minimum speed is shown at approx8mb depending on line condition your distance from the cabinet is limiting your speed
He should be getting speeds within the estimate, the distance is taken into account when the estimate is given by the checker otherwise it would give the maximum speed everytime. Also his noise margin is to high.
Many thanks for all your help, I will, in the first instance, raise a fault with BT, but I assume they will ignore me until the 'bedding' in time has taken place?