Don't be so stupid.
If you do well in your A Levels and go to a good uni but struggle to get a 2.1 then obviously you're not as bright as you thought you were, rarther you tried super hard at A Levels. If you have an okay amount of intelligence, but a very good academic attitude you can do well at sixth form. So if you feel you need to go to a lower uni to get the 2.1, then obviously you are worthy of that uni. It should be that those at top unis find it just as hard to get a 2.1 than those at 'lower' ones.
On another note, some don't try very hard at A Levels and get the relatively good grades needed to get into a higher mid-range uni, they change their work ethic and do really well. Why should they be less valued to the employer? Employers don't purely want someone who is extremely intellectual and academic, if you bring unique ideals, experience, values as well as a fairly decent ok degree then you're clearly wanted.
People on here lay too much admiration on grades, you really do. My sister is going into ethical business working for this big company on sustainability, all her modules at uni were shaped towards this, she's a bit hippieish, confident and creative. Who would the employer want; someone from York with a business degree whose only values are his degree, or my sister, from Reading?
Firstly, who says that you thought you were bright just because you got good A levels? Successful people aren't necessarily arrogant about it.
Secondly, there are loads of non-academic reasons why people struggle at uni, just like there can be at school. It's a new environment and people get homesick. You can get depressed there.
You might have been happier in high school. You get 4 years to settle into your high school before sitting your first exams, and if you've lived in your high school town your whole life, about 16 years to settle into that town, and high schools are often smaller places which pay you more attention (at least they take attendance registers and phone home if you don't turn up to class). At uni, if you have winter exams, you can have exams counting towards your degree within about 3 months of getting there. They don't notice if you don't attend a single lecture all term. Therefore, it's possible that you could go into a uni exam not having seen any of the material you're going to be assessed on, whereas at school, this is very unlikely to happen.
Have you ever actually been to uni? I'm sure that if you had, you wouldn't hold your unusual beliefs about the value of uni grades as a measure of intelligence over school ones. Most uni exams that I've seen can be studied for, like a robot, using past papers and course notes, without showing any creativity or extreme intelligence. There are plenty of exams where it's the exact same questions, with different numbers, at the same question numbers every year.
Also, on the topic of your "hippieish, confident and creative" sister vs "boring grad with good degree". I'd say it depends what skills the employer needs his employees to have. If the "boring grad with good degree" demonstrated better computer skills, say, and the skills and knowledge in his degree were of importance to the job, then maybe they'd be hired over your sister. Especially if it was for some very professional role where things needed to be done "just right" and not left down to the whims of someone who is "hippieish and creative". Also, who's to say that your "hippieish and creative" sister might not suddenly change her mind about the job and leave? You might think that being "hippieish, confident and creative" makes her a more interesting or better person, but that doesn't equate to them being a more valuable employee.