Of course Etihad can operate into LGW with no problems despite having LHR routes. The market/catchment is much larger to tap into (Qatar Airways and Emirates also do the same!).
For PK, BHX and LBA may be profitable but it may be that adding too much frequency would render the route into losses; may be limited to an extra flight a week.
For AI, you've got to pitch the route at the right frequency to begin with and then increase steadily. Perhaps part of because of what they did and so still flock to LHR. Have a 777 if you want but don't be too surprised at 3 weekly services to start off with but using the 777-200s.