First, "Restricted performance" light and message comes on while you're bombing down the fast lane of the motorway:
Do not panic! In particular, don't floor the throttle to try keeping your speed high - it has the opposite effect, the engine refuses to respond and you slow down. Instead, feather the throttle to minimum that maintains your speed while you move to the middle lane. You are not restricted to 50 mph - in my diesel, i could reach (cough) at least the same version of 70 mph that everyone else was doing. What you can't do is open the throttle more than about a third before the engine management limits you. Ditto, there's a maximum rpm you can do, so you can still move off from stationary to reach 70 mph, but only accelerating slowly through the gears. In my diesel the limit is 2000 rpm; I expect it's higher for petrol engines.
Second, total brake failure:
I had this plunging down the hill to our local motorway junction roundabout. I thought I'd hit the accelerator by mistake, as the car did not slow. Taking a big stab at the brake pedal showed the brakes were still there, just a long way away. You need virtually all the pedal travel before anything happens. This is the dual-circuit braking coming into play: the majority of the pedal motion is failing to move one half of the system; it appears at some point in the pedal travel the two circuits (which are normally connected to balance the braking effort) separate, and then you can operate the remaining half.
Note that the brakes will be at most 50% effective, as the split is front-left-rear-right, and vice versa, for stability, though there will still be some brake-steer. I presume ABS won't function either.