Want to move to London but need some guidance

Long story short, I had lived in the UK for 6 years, got settled Status but I had to move back home to Bulgaria for a couple of years. Last year, I went back to the UK for a couple of weeks, and seriously started considering coming back, since despite being financially okay, my social situation in my home country isn't great. I found it a lot easier to connect with people in the UK. Back home, I found out quickly that my "family and friends" took advantage of me, and they don't acknowledge my existence unless they need me. Meanwhile, I still keep in touch with my friends from the UK, despite not having seen them for a couple of years.

So here I am, starting life from the very beginning yet again... I set it as as my goal to leave the country but I couldn't decide on where to. In the end, I made the decision to go back to the UK and give London a go.

I had only visited London for a couple of weeks in total, since I spent most of my time moving between Bristol and Cardiff for different jobs. I graduated uni from a former poly. My degree is in CS and I have a couple of years of experience under my belt as a junior developer, so I will polish my CV and start applying for jobs in London.

My question is, what are the more affordable relatively safe areas to rent a room in a shared house? I assume I will have to look Zone 4 and up. I have savings of about £20 000. Let's say I don't find a job immediately and it's easier to apply from the UK than doing it abroad: will the landlords be okay with me paying rent 6 months in advance?

As for jobs, if I can't find a job in my field, how's the market in general? I don't mind doing some menial work while I get my feet on the ground.