"considering I haven't done too well"
I do not know what your percentages are for grading but if you had continued the way you had and would have got even a third I would seriously consider mentioning it when applying/in interview (do it right and if the personal stuff is not likely to be petty nonsense in their eyes* that would not hurt); it is not that you already know part of a subject but that you can deal with university itself is what the would be interviewer will be looking for and that it would be the second year means that is even better (first year is usually so much getting everybody up to speed). Think back to how many people dropped out after a few weeks, after the first Christmas/exams and then those that did not come back for the second year because they could not hack it and I bet a few of those did stunningly at A level and likewise how many people that got on well despite not caring for the A level system.
*I broke up with my significant other probably does not count but someone getting seriously ill/dying or serious financial strain on families and you still managing to get something done counts for an awful lot.
"When I said I want to do IT/Computer Science".... I have yet to see a high school that can reasonably claim a good option for IT let alone appreciating what it can do when taken a bit further so I would not read too much into that. Oh and if you had not figured it out already (and the tone of your posts says a meeting with one of your former advisors would probably end in your delivering a headbutt) school careers/university guidance types are so very rarely any good.
Medic wise if you are really not sure what you want I would possibly consider doing a year of nursing training (my preference there would be towards the academic nursing degree options but that is preference rather than something to be considered solid advice) as there are serious options to branch from there although that probably means another year of student loan (this said there are some serious grants for nursing) although you will probably have to pick between mental health and medical. There is also the likes of operating theatre technician but I do not know how you would branch/move up from there.
IT wise there is little for general training that leaves a vast array of options open after a year or so but for the most part the first year will be similar so you might have to promise to read a book if you find another specialisation but you can probably switch up there as well, this will depend upon the university you pick though as they might not have so many options.
Combining the two... medical imaging I guess spent a lot of time doing IT and electrical engineering on top of the medical stuff although I do not know off hand how far that goes into the clinical side of things.
Edit- can not believe I had to include it in a second post and I do not have anything I can vouch for medicine wise but IT wise watch say
this video series and that will probably give you a reasonable indication of what IT will be like (assuming you have the time if you have watched a few in a row instead of getting dinner or something then signs are good).
