I have an (UK) ordinary degree in mechanical engineering. Is it worth the paper it's printed on? I've had absolutely no luck with job hunting. After months of applications, I've only gotten two interviews (both last year, before the pandemic), and the jobs I got interviews for weren't even vaguely engineering jobs (one was just CAD doodling, the other was basically tier 3 customer service)

I understand most people have honours degrees, so the fact that I can't say I have a 2.1 or whatever is an obvious red flag.

Is there any future down this path? Is this really such a huge red flag for employers that there's nothing I can realisitically do to make myself employable in my field? Is there even a grad programme that would take me so that there's a chance I could get a degree that doesn't seem so worthless?
It sounds like you're under the impression that fancy degree = job. That is incorrect. Fancy degree is nice and all but it's only the first cut. You will need to back your degree up with actual job search skills as well as technical skills.

For the former, this isn't the right Stack Exchange (use The Workplace). Read up on all the CV-writing, cover-letter writing, interviewing techniques, etc, that are necessary to get hired. Also be sure to make use of your alma mater's career center, if they have one. They'll have not just job listings, they will usually have counselors who will help polish your CV.

For the latter, I hope you didn't slack off during the degree because there's no substitute for it. If the interviewer asks, "here are two vectors, what is their sum?" you need to be able to answer because if you can't, then the rest of your CV is called into question. Make sure you know the basic coursework, and if you can keep up-to-date with the latest news in the field, even better.

Job searches are not easy. Unfortunately, you'll have to help yourself, because society doesn't owe you a job and nobody else will be able to help you pass the interview. On the other hand, they can be done: after 8 months I'm pretty sure many of your cohort will have found a job, and if they can do it, you can too.