I'm a corporate lawyer at a big law firm and I can give you a summary of what I know:
There are very few firms that pay the big wages and those jobs are hard to get. So when you choose "law" as a career, you are rolling the dice. If you are Oxbridge with good marks, you've got a decent shot. Others are playing a lottery ticket.
You will need to spend two years in a training contract which is a much smaller salary (and you are going to be in London working long hours so a reasonable commute is a must). And many people will make even less as a paralegal for a year or two if they have to wait to get a training contract.
Once you get a big law job, it can be a tough ride for some people. The working hours tend to be long and the work isn't fun. The people you work with are often stressed out and will take it out on you. The hours of the job isn't even the worst part - it's being on call 24 hours day, 365 days a year. I had a client have a full blown meltdown because he called the office at 1 pm on Christmas even and no one picked up. The office was closed because not only was it Christmas eve - it was a bank holiday that year because Christmas was Saturday. I heard him yell all of this at me whilst at my parents house, supposedly on holiday.
The corporate law track is "up or out". You get auto promoted every year so either you learn what you need to be useful at the next level or they will fire you. It's brutal. Not just that, but it takes 5-10 years to master any given practice area. So if you are let go (either by firing or by redundancies) and you haven't mastered your skill, you may be completely out of luck in finding another job. For the first 2-3 years you are especially vulnerable. You know nothing so you are the mercy of another law firm needing another newbie or else you are unemployable.
Even if you are doing well and learn your job, few people make partner at law firms. Big law firms are highly leveraged (with about a 10-20 associate per 1 partner ratio). This means if you look around at your peers, only one in 20 of you are going to make it to the top. My starting class at my first firm had 33 people. One person made equity partner 8 years later.
There isn't a great job market for the people who don't make equity partner. They need to be prepared for major salary readjustments.
The people that make equity partner will be under huge pressure to win clients and business all the time. So many of them are alcoholics it's crazy.
Anyway, that's the end of my rant. The only reason these jobs pay so much is that no one wants to do them.