Alas with FTTC the speed you get is largely determined by the length of the line between the cabinet and home.
The original 40/2 is the headline speeds i.e. those closest to the cabinet will get.
Half way down the page is a rough distance/speed guide. From your post does a distance of 1000m (1km) sound reasonable for line between house and the green fibre street cabinet?
All the estimates given are connection speeds, so yes speedtests will very often be lower. Though to date most people getting FTTC have got speeds above their estimates, the key word though is estimate.
You can try and exercise consumer rights and walk away, but to be honest no other FTTC provider is going to do better as the physics are the same for them all.