As someone else stated, the biggest problem will be doing the practical coursework or practical examinations. I would suggest organizing that with a nearby school. As for the A levels, you should study Maths, Further Maths and Additional Maths these will enable you to take almost all the available Maths modules at A level. I assume you are good at maths or are willing to study hard, because these will not only give you an excellent preparation for a maths-based degree but are also much easier to study than other A levels as there is no practical element and I think you can avoid the few modules with coursework.
In fact, if organising the physics practicals turns out to be too difficult then you might consider not studying physics but instead just taking as many maths modules as possible and writing to the universities you wish to apply to explaining your situation. The mechanics modules cover most of the physics you will need in engineering in any case so I would imagine they would be quite likely to accept you.
So yeah, try and organise taking the physics practicals, and if you can then study physics as well.. if not then just study lots and lots of maths modules. I think you need 3 modules per AS and 6 modules per A2 (obviously the ones you do at AS count towards A2) so if you were able to do Maths, Further Maths and Additional Maths for full A levels then you would take 18 modules in total, I think the pure modules are compulsory though.
The actual content isn't very difficult so long as you are willing to dedicate the time to practice and learn it :P And the decent supply of past exam papers is incredibly helpful, just do every single past paper in the months leading up to exam season and you should do fine.
I just finished my A levels and did Maths, Further Maths and Physics alongside Bio and Chem all through to A2. I wish I could have taken Additional Mathematics and taken more mechanics modules instead of studying Chemistry but my school didn't give me the choice :(
Seriously though, use websites like
http://www.khanacademy.org to supplement the textbooks (use the textbooks for your exam board as this is a good way to help get high marks) and get some revision guides to help you go over stuff and you should be fine! The content isn't that hard tbh, I was able to teach myself most of Further Maths as my school was unable to timetable many lessons in it and I got an A so you should do okay!
And obviously if you get stuck on topics in particular you can use PF for help! The people here are really friendly and very smart indeed!