A young law-hopeful will face insane competition to get the relevant work experience and eventually get on a vacation scheme (2% success rate) [1]. Here they will do 2-4 weeks work with the firm they want to apply for.
Next, they need to apply again to receive a full training contract at that firm. Competition for these is even tougher (~0.5-1%).
Now, as part of the graduate intake (training contract) this person will be one ~100. Here are where well educated, nervous, fresh-faced young staff become the firm’s best workers - it's basically a two-year job interview. At the end of the two years, around 70 will be hired by the firm as new lawyers. From there another 20 will parachute[2] out per year. Of my brother's 35-person intake at a magic circle firm only two remain seven years on; many are in completely different fields. Around one person per intake will make it to the top - by this point they are the 1% of the 1% of the 1%. The entire business is structured to concentrate wealth in the hands of as few people as possible, with ludicrous hourly rates that junior staff never see, rigid seniority levels and the illusion of prestige.
Source: Brother at mid-sized magic circle firm + my own 3 months' subjective experience at two top-tier law firms before I decided to jump ship (I'm biased!).
[1] http://allaboutlaw.co.uk/law-careers/vacation-scheme/vacatio....
[2] A popular term in the field referring to the ease in which a lawyer working at a prestigious firm can move in to something less prestigious and a whole-lot more rewarding somewhere else. I met people applying for top firms with the sole intention of doing this.