Getting a job and quitting PhD at university in UK
You say "no one needs to know about my PhD drop as my official graduation took place in 2018" but surely your CV would indicate that your Master's finished in 2017.

Anyway: I can't advise you on whether it is a good idea or not to drop your PhD for industry, since that is a very personal decision. However, I would note that in most UK universities that I know of (and almost certainly in a Russell Group university), the first year of a PhD has a kind of "probationary" status, at the end of which a student would be upgraded to "confirmed" status if all parties are satisfied with the student's progress. Therefore, withdrawing/transferring within the first 10-12 months is a bit easier and less problematic than doing it after 12 months; if the people in your department are sufficiently mature and professional, then they should accept that you have genuine reasons for your change of path.

You also ask how long you have to inform the university of your withdrawal. I would say that you should talk to someone ASAP - if you are not confident doing this with your supervisor, you should try to find the "Postgraduate Tutor" or similar person for your department, or even approach someone in the Faculty of Science. They can advise about practical matters such as terminating your registration as a student - universities nowadays have to be more machine-like in dealing with changes of registration.

BTW, you don't specify if there would be any issues about e.g. immigration status. I am assuming that there are no such issues, since the people who offered you the job should have discussed this with you; but if you are on any kind of visa or study permit then withdrawing from the PhD would affect this, and you would need to talk with the prospective employers about what to do.