On which website can I find good authors in a specific field? For example I want to know good British authors who write about computer science (more narrowly about operating systems and Unix).
Unix for Users is apparently very readable and British English, one of the writers lectured at Leeds University (though it's quite old).
As far as C is concerned, though the writers are American and Canadian, this is the go-to book and very readable:
I pushed back and asked again for British English, but it seems it's the bible for C.
And in no way to be patronising, the Dummies books for all your needs have come highly recommended and I would agree with this. I used them when I was in I.T. because they make no attempt to over-complicate a subject in order to appear clever. I imagine many of the authors are US/Canadian but the requirement to keep the writing straight-forward in the Dummies range should make them readable.
The other recommendation is to start your journey with Python on Raspberry Pi. Apparently a cheap platform that's easy to learn and get code working. The consensus was that you'll learn far more from this than any book you pick up regardless of the language/author.
Without knowing what stage you are at, I cannot say how helpful this information will be, but hopefully it will point you in the right direction. If I get further recommendations, I'll edit this answer.
You could have a look on somewhere like www.upwork.com and search for British English writers and technology if you're looking to hire someone.
The site won't tell you if they're good, but it has a success rate for the jobs they have completed from which you can gauge their work.
There may be other similar UK sites too, if you Google site:uk online freelance jobs.
Can you give a bit more detail about exactly what you need and why?
I'm a British English ex-computer engineer turned author and worked on Sun Microsystems unix systems but it was a VERY long time ago and I don't write about computer science in my fiction. I quit that career for a reason!
Without knowing a bit more about what you need, I'm not sure how else to help.