Is it normal my 2013 ford focus st burns 11.6 l/100km?

Go by the average, not the instantaneous consumption. When coasting, it says you're putting gas back in the tank. For the hp it makes, that fuel consumption might be good. 252 hp isn't free.

First off, stop using premium when regular is fine. And stop paying attention to that thing on the dash.

Check the consumption by the old-school method, top off the tank, reset the trip meter, drive, top off and note the trip meter reading,

No.

That's around 24 miles per gallon (UK).

You should be getting more like 40 - 50 (7 - 6L / 100km) depending on the engine. The only way it should be that low is if you spend all your time stuck in traffic queues.

Is it normal? If you do a lot of city driving, like to accelerate quickly, and/or let the vehicle idle extensively, sure. But if you drive normally, no, it's not.