Originally Posted by
Wygun
Then, depending on whether your short haul flight is inside or outside the Schëngen zone you should head to the Dali (T4) or Velázquez (T4S) Iberia lounges, respectively.
Generally correct, but there are exceptions. Most IB-operated flights to LHR, and also BA-operated flights to LCY, depart from T4 (which also has a non-Schengen gate area, H1-13 IIRC) rather than T4S. IB flights operated by widebodies will usually go from T4S, though very few would go to Schengen destinations.
I would agree to go to the Dali or Velazquez lounges as appropriate, but with a 10-12 hour layover I would also be heading into the city.