I totally get where you are coming from with your fear of not feeling quite good enough when you are *sure* everyone else did much better or that they are going to find out about your ABB. That said, your A Levels are truly not a marker of how you will perform at University. Some people rise to the challenge and some do not and yes, while A Levels obviously indicate academic ability to some extent, they are indicative of your achievements at a certain point in your life - which as you have said you have mitigating circumstances for - and so, I wouldn't and you shouldn't be so hard on yourself.
I too was expecting better A Levels. I was on track for A*AA and then bombed in my finals and ended up with BBB. I was, like you, devastated, and, like you, had mitigating circumstances, but I still got into my first choice University and went on to secure a strong First. I actually have friends who got AAA for A Levels... A*A*A at A Level and even A*A*A*A*, but most of them went on to get a 2:1. I find life balances out and that because I didn't do as well as I wanted in my A Levels, I worked harder over my three years at University and had the drive and determination to do so.
Firms
will take this into consideration. I have spoken to many firms now and when I explain directly to HR about my A Levels and then round it off with 'oh but I do have a First Class degree' and tell them all my modules grades and how they were all firsts, they tell me to apply and that they want people who are hard working, different and driven. Do your research, apply smartly and you might actually surprise yourself with what you come up with when TC applications come around
Ultimately, you are going to Oxford! As everyone else has said this a fabulous achievement and you should be proud. Go in there and prove to yourself you deserve to be there, don't opt out before its even begun!