I broadly agree with this and just thought to give it a boost and add my two cents.
Looking at data published by
Chambers Student and
AllAboutLaw, there are approximately 5,500 available training contracts
total in the U.K. every year. In the Chambers Student 2020, there were 82 firms listed. For argument's sake, let's assume that these are the 'top-82' (however you want to define that) commercial law firms in the U.K. In total, these 82 firms offered approximately 1,357 training contracts. This means that over 75% of training contracts in the U.K. come from outside of those firms!
Now, whilst it might be rich for me to advocate applying to smaller firms/regional firms, etc. when I didn't, I do think candidates give themselves a tough time when they
only apply for the highest bracket, and don't even consider those who aren't (for argument's sake) in the MC/SC/Elite U.S. bracket. I knew of an associate at an Elite U.S. firm, who trained at a low-mid market firm. They then moved to an SC firm, and then moved to the U.S. firm.
I should add that you don't have to make those moves! You should focus on doing the kind of work you want, at the place that you want! Prestige shouldn't be your main focus in this day and age - that will not make you (or the firm you work for!) happy. Rather, you should focus on your attraction to the career, what kinds of work you like/would like doing, and what environment is best for you. Likewise, some might say that they know the MC/Elite U.S. is the best place for them, which is equally fine and valid.
Anyway, I think the main point is just to echo what
@Matt_96 has said, and encourage candidates to focus on the career and the work ahead, rather than to get bogged down with obsessing over one firm, or 'prestige'. And, if that one firm/prestige really matters to you, there are countless examples of individuals who have worked they way up and achieved it - that path always remains open.