If you are part of the IATA interline agreement then you can get from LGW (London Gatwick) to Split (SPU) or Dubrovnic (DBV) in croatia for £65 on an ID90 ticket inc tax with BA. 6x week to DBV 3x week to SPU.
There are usually only 3 cabin crew so there should be 1 or 2 cabin jumpseats that you can use with the captains permission if the flight is full. I think UK law prohibits the use of the cockpit jumpseats other than that airline's staff, and so you are limited to any seats in the cabin.