I don't think you have too much to worry about regarding the offer from non-russell group. Lots of top firms don't look exclusively within russel groups as long as you can prove yourself really. A Masters at Oxbridge, for example the Bachelors of Civil Law (BCL), would be very very appealing no matter what Uni you did undergrad at, as most other applicants for firms would likely not have that. And since, atleast to my knowledge of the BCL, they are extremely intense with the workload so it demonstrates you're capabilities. But it is not the be-all and end-all since firms have come a long way with hiring practices. Hope this helps.