It's defo worth ringing up your department's admissions team to explain your situation and see if they'll let you attach a supplementary engineering-related statement. You've got good predictions for Maths, FM and Physics; so I wouldn't say you're horribly disadvantaged wrt academia.
But yes, defo be prepared to state your case a fair bit (interviews, etc)...and make the phone call BEFORE you send off your app. Make sure you note which admissions team member you're speaking to, and what they said, so if you do apply and they suddenly decide to turn on you (hopefully they won't though), you'll have solid proof that someone reassured you it's okay/not okay, etc.
I'm sure they'll understand that people do change their minds wrt uni - after all, recently in council we were discussing UCAS' idea of PQA (post-qualification admissions) - one major point that came up was that it's way too early for students to decide what they want to study before they're ready/well-informed/have their grades, etc.
But yeah. Worth a try, phone-call wise first anyway.