Cannot use a GUF2 nor a Joker on BA/IB flights. As you say, JetTime was a wetlease, IB is a proper codeshare but only uses a BA flight number for IT purposes.
The IB flight numbers in the 3xxx range is standard for IB - almost all international shorthaul flights are in that range, 6xxx for longhaul. Real BA flights to MAD are listed as IB7xxx.
Have you looked at the LCY-GRX flight?