We have reserved a flight through Iberia (better price) putting us on a BA flight from EWR to Heathrow in business class.

I understand we check in at the Iberia counter when we arrive at the airport. Which lounge do we use, Iberia or BA in EWR and on the way home at Heathrow? Any tips on the lounge we get to use that we should be aware of?

Thank you for your assistance