Is there a way of selecting attitude mode when flying outside - ie even though the drone has good gps signal?

Not designed in, no. It has been suggested elsewhere that a modification can be done to the RC FW, but I’m not sure where I read that

Hi. Thank you for reaching out to DJI Forum. With regards to this matter. The attitude mode is a safety feature that is only available when encountering weak GPS signal during flight. It is not advisable to fly the drone using attitude mode since the drone may be more easily affected by it's surroundings. which makes the aircraft hard to operate. If you have further concern or inquiry, please feel free to post it here at DJI Forum, we are here to help. Thank you.