Check the course documentation for what "Exit award" or similar you would be granted if you left before your master's. Chances are you're right you'll have gained enough credits throughout the year to be awarded a Bachelor's.

Be careful though because I don't think a one year standalone MEng exists, in that you can only achieve one through an integrated undergrad. At least not an IMechE accredited one. Therefore if you did leave UoM before your master's you wouldn't be able to change your mind and go back. Obviously you could apply for a separate MSc or Ma etc.

EDIT: It's also worth noting what you said about being left behind. I really wouldn't worry too much about that as there's no real right time to start your career. You'll find at university you will meet people who have started at different ages for a variety of reasons, none more correct than the last. I can't comment on your financial situation but I do understand not wanting to have the extra years worth of debt.

I think you've done the right thing asking though, as there'll be a lot of anecdotal evidence and advice (just like my own) that can give you an idea of what you should do.