How much assistance do we get for extending agle children? None.
Actually that isn't quite true. At ks3 we have more advanced work sheets (yawn) for the more able kids. TBH I seldom use them, and prefer to use more open ended tasks which the more able can run with....for example on a poster activity on 'Noise' and 'Sound' the more able boy in the class did his posted and then calculated the speed at which the sound of Krakatoa moved round the earth!
My more able kids at KS4 get sideways extension but more often I give them the A level treatment of the sunbject...but this is driven by their interest in the 'real' answer as oppsed to the GCSE version. I am keen to make the more able integrate all the sciences, so to explain why you get hot when you exercise, based on what is happening at a microscopic level in the muscles, linking Physics and biligy (and chemistry)
I also do lots of cross curricular stuff for the more able, make them write science haikus, researc Pavlov when we do the reflex arc etc.
But I get no assistance for any of this at all, most of it is off the cuff stuff that I just know.
My dd seems to be very able, at least at the moment, who knows what will happen in the future. I am far more keen that she can work with her peers and do lots of out of school activities like brownis, music and karate. From what I have seen of the most able kids, the best (and nicest and most well integrated) have lots of hobbies to keep them stimulated.