i would say if you're wanting to do masters/further research, a good thing to do is find a uni that you could happily stay at for the entire duration of all courses. do they have research in things youre interested in? is the masters good? because its basically easier to get into masters/phds if you already know all the staff, and are friendly with them, and interested in their research. basically, could you see yourself there for a long time, do they support research, do they offer research asssistant schemes, are the lecturers people who take on phd students, is the research of interest to you?
i'll be honest it doesn't hugely matter about the ranking, or about it being a russell group. if you will like it, thats all that matters.