Do you think it could be worth putting "2:1, English" and not state the university for your shop work applications? I don't know whether it would work or not.
Arts degrees definitely aren't useless, they teach lots of soft skills and can certainly be valued by employers.
The university have a pretty good careers service and you might well find that there are more jobs about the place than you first think. There will be local companies in most areas of the country and as you're looking at retail etc, it sounds like you've got the capacity to take something where you need to work your way up.
www.careers.cam.ac.uk/
They can help you get your CV set up and will even do practice interviews with you. It also has a whole website of job opportunities where people are especially keen for Cambridge graduates.
I understand that being told you're doing a good job is a valuable thing in a job. At my work we really make an effort to say thank you to each other because that makes everyone's days better and costs nothing. However, I would say you can't necessarily expect it in a job.
If you had a think through, what would you say you're good at? Do you feel you struggle with interpersonal skills (from the middle that seems to have happened here?) What things do you think you would enjoy in a job?
I'm really good at taking technical things and distilling them into a form a lay-person can understand. And at taking the key points from a long document to enable people to apply what's within. Other people are good at other stuff, but having a think about what you enjoy can be valuable for ending up in a career you care about.