First, I have no idea how to answer your question. I did do some writing, but nothing famous and not scripts. I was a journalist to earn money while I was went to the university.
But, your question made me curious. It really does look like the link in the sibling post is as valid a link as you're going to get. I'd suggest your SO put the work in and make use of all the resources at that link.
Having said that, write and network. Certain jobs aren't acquired by the usual methods. There probably is no long interview and application process. Instead, your SO will have to do something to get noticed.
There are probably lots of ways to get noticed that involve luck. I'd not depend on luck. I'd make opportunities to get lucky, however.
I did find this link, it seems like a good link and the information passes the smell test. Here:
https://www.theguardian.com/careers/insiders-guide-to-becomi...
I think the networking idea is probably one of the biggest ways to get noticed. Join local writer's groups. Go to workshops. Get to know people. Instead of contacting them and saying, "Hey, I want a job!" Have them contact someone, probably having made some good contacts first, and say, "Hey, I really could use some advice."
I'd suspect that getting contacts is the first step. Do the research. Make use of the site they sibling post gave you. Do what they suggest, and then some. If there is an event for screenwriters, be there. If there is a pub they frequent, be there.
I'd not immediately pressure for a hire, or even to show samples. I'd listen, learn, ask as many questions as I could, and I'd make it a point to listen. People love to have an audience. So, when developing this network, be an active listener. Active listening is a very, very underrated skill to have. People love to talk, encourage them to do so. Pay attention to what they say. Ask good questions. Ask questions that invite them to share their knowledge. Most of all, truly listen and take their advice to heart.
Write... If you want to be a writer, write. Write a lot. Write all the time. Write about anything and everything. Write stuff that people will see, even if it is just forum posts. Write constantly and cover a wide variety of topics. Write about what you know, and use your writing as an excuse to learn more so that you can write about that.
Put yourself out there. Find people who will honestly give you advice. This means you're going to need people who understand both good writing and the subject matter. You can find those people online or in the real world. Friends and family shouldn't be your first choice. They aren't always able to be unbiased. Nobody likes to tell a loved one that they suck and their dream is hopeless because they have no skills.
If they do suck, that doesn't mean they can't learn - so keep at it. Keep writing, keep taking classes, keep going to creative writing groups, go to any writing group that will have you. Write ad copy for free. Find a website and edit their work and then send it to them.
Build up a portfolio. Even if it is just a blog, get your stuff out there. They want to see someone that can be prolific and good. They want to see someone who is capable of putting out consistently good work. They want to see someone who has motivation to complete projects.
So, write, write, write... Learn, learn, learn. Network until your schedule is so full that you barely have time to live a normal life.
And be ready for rejection. Be ready for a lot of rejection. Be ready to sacrifice your dream and be willing to write for a different company. When rejection happens, and it will, don't give up. When they reject one script, ask them why and then make the script into something that fixes those problems.
I'm sure scads of people also want to write for the BBC. They not only have to be better than the rest, they have to have more drive than the rest. They have to stand out and be noticed. They have to have someone who is rooting for them on the snide of the company. The BBC has to want then, make them want you. Make them aware that not taking you means that their competition will take you. Make them know that you're an asset that they don't want going to a different company. Make them know that you can solve a problem and fills niche that nobody else can fill better than you.
Some of this is covered in the link I gave you. I could go on but this is already long enough. If they want to work there, they are going to have to work at it. Use the resources available and be ready to work harder than your SObhas ever worked in their life,