I am an employment lawyer, and a partner not magic circle, but big firm. £400 per hour is less than some of our partners in London charge more than some others, the other rates are comparable.
I have once (in over 15 years) done an unfair dismissal case where the costs were greater than £100k. It took 3 years from start to finish and had a lot of other issues involved. It was not a good use of money if this was just a decision about whether the costs would be cheaper to fight or to settle, but that was just one factor.
I also deal with finances for my firms employment department generally, and at no time (including bankers bonuses and discrimination cases) have we charged £200k. Even for very complex disability discrimination you'll never work again cases.
I can understand why you may be aggrieved, but I do think you are being unfair in various ways. For a start I just cannot see how you can have run up those costs with no warning. I would never give a client 200k credit, I am just not allowed to, and I would stop my other partners incurring those charges without money being paid and money on account, so presumably you had interim bills and paid at least some of them. Presumably that gave you some idea that the estimate was being exceeded and updated that estimate. And this was discussed. If it wasn't you have a ground to complain.
I also assume your case had discrimination and equal pay elements, that does make it harder to fight and more expensive, and again I assume this was explained.
However you do say that the solicitors didn't take various steps which had been agreed/were necessary at an early stage and that is disappointing, but that doesn't mean that th ewhole profession is deserving of a slating.
Personally I try not to act for employees very often, as they are always difficult in employment law cases as the costs aren't recoverable, and you can never dictate the response of the employer. For example they may have been unwilling to mediate/settle at an earlier stage and deliberately acted to push your costs up. I have a number of big clients who may sometimes do that, in effect to outspend the employee. That is usually outside your control.
I think there are two sides here and we are only getiing one.
(by the way I aslo do various jobs on fixed and other fee arrangements, but I will not usually act at a loss, which can often happen if an inidivudal is unresonable, demanding, doesn't listen to advise etc and that is often why I chose not to act for employees. when I do act for them sometimes I do so pro bono, but again I can't think I would ever do so for a claim worht by your own admission more than £200k)