League tables can be very misleading as different league tables actually measure different things. There is no definitive listing, however, speaking with academics from around the world, the Russell Group comes up time and time again.
Ultimately, poly or no poly, your future career will depend on continued postgrad study as this will push you down one of two routes a) academic and b) profession. At some point, the two can and will merge. Until then, you would have to make a decision on your priority.
Either way, you will have to study in some form or another for the rest of your career and your bachelors wouldn't come into question just so long as you could show an employer that you had the aptitude and attitude that they require of an employee.