Wonder if anyone is able to advise on this. I’m trying to book LHR-EWR in Y (Nov 5th) then JFK-LHR-ARN in I (Dec 20th)

I see on ba.com using multi-city the outbound flight for £385, the inbound for £927 - but when I select those two fares it reprices to £3000.

I tried using matrix ‘+CABIN’ but it didn’t bring up the fares either.

Is it possible to book multi fare, multi city bookings and how do I go about it? Thanks all!