One mile before Junction 9 there is a large blue sign that says 'Junction 9, 1 mile'. There's another one at the half-mile point. These are remarkably handy for ensuring that one turns off at the right junction and they have the added benefit of avoiding having to scratch around trying to find a post-code for the junctions that don't have one.
I believe these signs have existed since way before the invention of the microprocessor, let alone the satnav, and can therefore be relied upon absolutely.