To clear up the confusion for non-europeans: Schengen and EU are two different things. Many EU countries are not in the Schengen area and the UK is one of them. They're in good company: all the former communist states in the east that joined the EU are not in the Schengen area either.
The B and C concourses, and the upper floor of D, are inside the Schengen area. E, F, G, and the lower half of D are not.
They're building a new H concourse for discounters like easyJet. I'm not sure how that's gonna work out in terms of separating the Schengen and non-Schengen pax.