I have taken codeshare flights from/to DUS several times when connecting at LHR on a CX itinerary and have booked codeshares from/to TXL later this year.

Scandinavian destinations like ARN, CPH and OSL also get BA/CX codeshares but the lounge infrastucture there is limited compared to the BA Terraces at German airports.

Would like to think also Mediterranean BA destinations have these codeshare flights, but not sure. Best to check with BA or CX.