I've never heard of DELF or B1 either, but if there's a website for the qualification, it might say what it's equivalent to, or they might be someone you could email to ask about it. You could also email the undergraduate admissions officers at the unis you're thinking of applying to. If the qualification you have isn't appropriate, you'd have to do Greek, Greek combined with another language from scratch or another non-language subject combined with a language from scratch. It's extremely rare to find French offered from scratch or even post AS level though, and most good unis ask for an A-level in any modern language if you want to start one from scratch, so you'd need to find out if they'd accept Greek.