The universities don't let you decide if you are good enough because people's circumstances, support, self-esteem and confidence, etc varies between people and over time. If they both gave you an offer, it meant they both thought you were good enough for their university, bearing in mind they have grade inflation to look good. I really doubt you have the lowest grades. They are not charities, they would have given your place to somebody else if that person was better. Universities sort your skillset out in the first year. Do you really think QM is going to be pissed off, if you come back and say you made a mistake, I mean, maybe, if you plan on being rude, otherwise absolutely not, they just see £££. Enrollment probably takes a week. Which uni is better is irrelevant, the fact you are still sleepless about it, after going through all your excuses, to me, is a good indication that you know you have fundamentally made the wrong choice for you personally.