Actually, there are some distinct differences in the packaging I noticed using the EL a few weeks ago. Some stations have seperate barriers (one set for Underground lines and another set for EL - even though in some cases they’re right next to each other). Which is more like switching to between Underground and overground services. But EL uses Underground fares within the Zones.
The new western entrance to Tottenham Court Road station just has the purple EL signage above it, whereas the refurbed eastern entrance has the traditional Underground signage.
Unfortunately, I didn’t catch the EL to/from LHR (it wasn’t convenient for where I was staying but there is an £7+ airport surcharge that the Piccadilly line doesn’t have.
Either way, it’s an excellent addition to the network.