You’re eligible if you’ve applied for Universal Credit but haven’t received your first payment yet. As long as you can prove you’re in financial need—like you can’t afford food, rent, or you’ve run out of cat food (that counts as a legitimate crisis in my book)—you’re in.
Even if you’ve had an advance before, you can still ask for another one. The system is forgiving, like that friend who always loans you a fiver and never asks when you’ll pay them back. But please, be nice to that friend.
There’s no credit check. Your credit score is safe. The government won’t judge you for that one time you bought an inflatable unicorn costume on a whim. They just want to know you’re in genuine hardship.