At 32 I think you've left it a bit too late. By the time you complete your engineering degree and achieve any form of valuable experience, you'll be competing with engineers half your age and twice as smart.
The quickest way is for you to go through the BTEC route. I.e ONC followed by HNC. The ONC will cover subjects, including Maths at GCSE in the earlier stages. So you don't need to do any more GCSEs.
Try and get your employer to pay for your part-time studies. If they agree they might even offer you apprentiship or training.