Hi Jay,
We share similar situations. I am also about to turn 29 and have not as yet commenced any meaningful career. I am also looking to study Computer Science (in Birmingham, too! Are you me?)
Firstly, I would tell you to put out of your mind any concepts of competition with your peers. Life is meandering and different for each one of us - you will have 40ish years of a career ahead of you if you remain strong and follow through on your current plans. You have already taken steps to take control of your life and it is in doing this that you will earn your own self-respect.
Graduating at 33 is perfectly acceptable. You will find throughout life that many people change paths and directions. Comparing your own life with others is fruitless, because you are not them, and you will never understand their motives and purpose. All you must do is understand yourself and take control of your own destiny.
Regarding Comp Sci;
You should get in touch with the provider of the Access Course to ascertain whether your current grades will be sufficient to gain a place on the program (my understanding is that an Access Course is by its nature designed to provide access, not deter you from progression. A little research can't hurt and will put your mind at ease.)
I have already undertaken an undergraduate degree in a tangentially related field and will be going on to study the MSci at Birmingham 2014/15. Whilst there is talk of offshoring within the Computer Science industry, if you are canny and observant of skillsgaps within the UK market (given that this is where you look to build your career) it will be entirely possible of achieving a salary anywhere between 30,000 and 60,000 (or more if you're lucky?) by the time you are 35. The industry is vast and continually expanding in new directions and it is up to you to research and determine which area you would seek to be employed within, the usual rules of networking and visibility (attending meetups/publishing to public repos/working on open source projects) applies as in any industry.
Finally, you should not look at your current path as your 'last chance of hope'. You have a lot of negative messages in your head, and whilst they might have helped you to formulate your energy and determination, you should seek to replace these ideas of being a late-starter etc with an understanding that you have taken the time you needed to fully understand yourself and will now enter into a career with the energy and passion of a man of conviction who seeks to prove himself, for himself. Not for parents, family, friends or society. This is a wider comment regarding life, but being happy in yourself is a keystone of contentment.
If you have any other questions, just ask :-)