Ask HN: What does it take to bill £500 a day?

I've done some work with Vue.js but the fact it doesn't appear in job listings that often pushed me towards picking up React. A few days into playing with it I typed 'remote react' into Glassdoor. Here's what I found:

- Listings offering £400-600 a day or salaries of £40-70k

- Lots of listings suggesting they're either remote-friendly or not opposed to full-time remote. Going remote with a London salary is a dream of mine.

- A lot of these appear to be strictly front-end positions, suggesting one can simply focus on React ecosystem mastery while barely dipping their toes into Spring/Rails/Django.

Now, I've only just started working with React and coming from Vue.js, it's very similar. At this point I'm wondering - how much experience does one need to demand that sort of money for working with React? What does mastery entail?

More generally - a question for those billing this sort of money - what do you specialize in, how much experience do you have, how could one get there in their mid 20s?