I also studied aerospace engineering and I also realised that it wasn't for me. If staying on for the fourth year really sounds like a year too far then I would consider graduating with the BEng, but then have a good look at the MSc options that would be available to you, you'll be surprised as to what varied non-engineering courses you are eligible for.
Having an MSc or MA in anything is certainly going to help establish a back up career, plus you get to focus on one preferred field as opposed to the broad spectrum of the MEng.