You could go back and do more A-levels (and GCSEs if they were required). Maths and physics would be the best choices, but product design, electronics (depending on how much technical theory you've already done) and engineering specific courses would also help you at uni. Maths is pretty much always required and physics just helps... Check specific universities for their entry requirements though.
Foundation years are another route if you don't have the right grades or qualifications. You will spend the first year of university doing the sorts of A-level material that they expect from their entry-level students. Some colleges can offer this, or you can do it at the university of your choice (if they offer it). It will cut into your student loan though, so you'll be able to do the foundation year and then 3 years to get a BEng on the student loan. Doing a placement year is highly advised as you can get experience (and get paid for it) in the third year. So the course will be 4 years plus the foundation, but the chance of getting employed goes up too.
Age won't be a barrier at university, and many people are 21 or older. It can make it a bit more interesting to do the GCSEs and A-levels though. Evening classes may be an option for the GCSEs. A-levels can just be done at a college, but you'll have to look into funding those too.