I would re-sit everything you can in order to get the A levels, what are your grades at the moment by the way in what subjects? The normal A level route is by far the easiest, so if at all possible i would really really try to stick to it. Maybe re-sit modules, or take new ones at college? At least you have a reason for not doing so well, and that will help you.
There are a few people on here who chose to do a BTEC, but im afraid i dont know much about that, and the safest option would be to contect each university individually and ask what their stance on the BTEC is.
As for the diploma, is that a degree? or something a stage lower? Because whatever you do i would try to avoid being a Graduate applicant as it seems to be the toughest route by far (and happily the one i took....)
However, at the end of the day (possibly excluding the BTEC route), you still need good A levels. As a graduate, i had a 2.1 in a really sciency degree and AAAAABB at scottish highers, but i hadnt taken physics or maths, and most unis wanted me to go back to college to do them, despite my degree. So my point is, i really think your best option is to stick at it just now and get the A levels and a brilliant amount of work experience and you will be ok!