Addictional Masters/Bachelors degree in Psychology after completing a Masters in Engineering?


I've been considering doing an extra degree in Psychology but I just don't know how to go about it in the United Kindom. I'm currently completing my Masters in Chemical Engineering (next year is my last one) and I still want to work within engineering after all my studies. I don't know whether to move on to a bachelor's degree in psychology or a masters. Master's has its perks because its not as long as bachelor's but I'm scared I won't get as much out of it as I could with a bachelor's degree. Is this the case? My course right now is an integrated Masters so I don't really know what a stand-alone Master's will be like (if anyone has any experience psychology master's I'll be so happy to hear it!).
Does anyone have any advice on this? Is it worth doing the extra years in a bachelor's degree if you've had no real education in psychology? I'm planning to take summer psychology courses though and read up a lot on it.
I know it's probably really weird doing a whole psychology degree when it's probably okay to just take a couple of online courses but I've got my reasons.
Thank you very much in advance!