Could you not stop your current course, ie, not go into 3rd year, and ask the uni to swap to another degree? If you have all the correct ALevels or equivalent to their entry requirements, usually unis are quite happy to work with a student to swap them, and depending on your skills etc, you could go straight into second year. Even if they made you restart year 1, that's only adding 2 years, rather than finishing a degree you don't enjoy and then starting another. Worth a call to ask.
If you know you don't enjoy your degree, why finish it to then start again? It makes no sense having 2 undergrad degrees, so if you want to finish your degree I would then do a Masters rather than start again. Plus, you'd have even more debts by starting this next year when you don't enjoy it or plan to use the degree in future. If you'd happily restart now I'd simply call your uni and others you would be interested in studying Cs at. So if you ask other unis if they have space on the courses and they do, apply now as there is still time, and see if you get a place. If you don't or can't swap at your uni, I'd do the Masters.
BTW, being 24 at graduation isn't a minus. It's a 3 year difference between you and the 'normal' graduate, and if I put you and 10 normal students in a line up, that 3 years wouldn't be visible. Employers look at your skills, interviews, cover letters, degree, not your age. Even in class, being 21 when you start wouldn't be noticed. Think of that 3 year difference as maturity, you've had more time to learn to work well with others, better time management and more time to improve skills. Some employers prefer slightly older candidates and some universities are the same.