Hi,
So I think you should look into different career paths that your able to get into with your current combination. Not sure about what scope the btec engineering allows you to go but the maths a level does broaden the potential career paths. However, if you hate the qualifications especially the idea of continuing it for year 13 then I would consider repeating with your changed subjects.
Also, when applying to uni after you repeat year 12 it does not affect your university application at all for any university. You are still going to take only 2 years to complete the qualification like most others doing a levels. Since you haven't done any exams too, you may not even need to mention this year on your ucas application. However, I'm not sure how the maths a level would be considered if you were to repeat the year again so I'd say contact universities directly for this.
The combination of a levels you mentioned you would repeat with opens you to alot of different career paths so I don't think you need to worry too much about that at the moment. My advice is to get some work experience over the summer in different fields e.g medicine, dentistry etc. This would help you see how the actual work environment is and whether you would enjoy the career.
Some other career options are; computer science, finance, maths and alot more. I think ucas has a career finder tool you can search through.
Good luck