We are flying from JFK to London Heathrow via Madrid with a four hour layover in Madrid. Both legs are on Iberia Airlines. Where do we go through customs? Madrid or, hopefully, not until we reach Heathrow? And coming back, LHR-MAD-JFK---- do we have to clear customes twice, Madrid and New York, or just at our final destination, JFK? Thank you.
Heading to London, the immigration check will be at Heathrow. On the way back your passport will be checked at both Heathrow and JFK, but not Madrid.
I'm curious about this - you will transit in London on the way there but I'm not sure if you will have to go through immigration in Madrid since the UK does not follow the Schengen rules (I believe). Customs will only take place in Madrid.
On the return, other than the transit in London, both immigration and customs takes place at the first port of entry into the US which will be JFK.