It is harder to get into Russell Groups than non-Russell Groups which means those who get in are of a certain calibre of student. Academic achievement is a keen reflection on someone's ability to retain knowledge.

A degree from a top uni is a platform from which to build on. It is not a guarantee of anything. However, a degree from a Russell Group/top 25 uni will say a lot about the background of an applicant and this will mean they are already at an advantage to one who has a degree from a lesser uni.