The issue is that lawsuits are often not obvious. There's almost always a non-trivial risk of losing. Therefore, in a universal "loser pays" system, it's even riskier for a poor (or even middle-class) person to sue a company given that they could be responsible for bankruptcy-level legal fees if they lose.
It was my understanding that pro-bono is under rate or free services, correct me if I'm wrong. I think the terminology you're looking for is 'contingent', where the client only pays a % fee to attorney if they manage to generate a judgement. In this case, does the UK insurance kick in to pay a fee to lawyer if they lose the case?
I'm talking about civil cases here, not criminal.
A friend of mine had a law firm take their (civil) case on a contingency basis and there was an insurance policy to cover the lawyer’s time in case they lost.
The simple[1] solution to that would be “loser pays if they spent more”. Which might seem odd at first glance (why punish the loser even more?), but would align the incentives in the right direction.
[1] Read: I haven't put much thought into ways this could be abused
https://slatestarcodex.com/2020/03/30/legal-systems-very-dif...
The article title’s a play on David Friedman’s book: https://www.goodreads.com/book/show/30066446 (I haven’t read this but sounds interesting)
Which is a risk worth taking if someone is suing for the right reasons and it's a once in a decade matter of major importance. That or they're fairly sure they'll win the case.