Many law firms recruit blind now, so it really isn’t such a big deal any more. Having said that, some research shows that even with blind recruiting, some firms ended up with more white, middle-class males from RG unis. Why that is I can only make assumptions:
1) Some of the best RGs offer lots of decent opportunities to get involved with law extracurriculars which can boost your CV (again though, this is becoming the norm for some non RG like Leicester & Lancaster, too). Their societies are usually strong and well connected too. Yes, it’s not going to get you that job, but people knowing you, and you possibly getting work experience out of exposure doesn’t hurt.
2) As the entry tariff tends to be higher, the students on the whole can be more driven, motivated, and well, tbh academic. That’s not to say there aren’t people like that at non RG, especially now that due to cost, many live at home and go to their local uni even with high tariff quals. But on the whole, those who do well at A-Level to get into a top university, unless they nosedive dramatically or get into a nasty habit, will probably do well on their course.
Ultimately, I wouldn’t get hung up about RG too much unless you are aiming for top 5. I did, and I’m not sure it’s made that much difference. I have friends at a mix of unis studying law (as am I), and I have to say, I think some of their courses offer more variety and practical experience than mine does. Some courses have a much smaller intake too, which isn’t a bad thing; high student numbers is a bugbear on my current course as it has meant cuts to quality imo. My uni is a very traditionally academic and textbook place; a bit dull at times tbh. My best experiences have come from competing in inter-varsity events, networking and the particularly brilliant student-led societies. The opportunities are usually there - make or break it with them.