(Caveat: I have no personal experience of the industry but like another poster here have a child at drama school and she knows a lot of people).
As countless posters have pointed out, the important thing is how to access the experience, technique and knowhow you will need in this profession.
Yes, some people do go straight into the profession without having been to drama school- I believe Rob James-Collier who played Thomas in Downton is one- but if you look at their bios they often have some other asset instead: years of experience in a good am-dram company, private lessons etc. Walking straight onto a professional stage without either experience or training is hardly likely.
Since the option of having already done 20 years in am-dram is clearly not for you, the first question must be whether you would be happy to go down the am-dram route now without necessarily planning for a profession. Some am-dram really is very good and could be equally satisfying. If not, then drama school seems the obvious option. There are all sorts of things you may not have thought about, like learning to breathe in the right way, learning to work with your body without damaging it. Drama schools provide training not only in acting, but also in industry-related aspects. They will often help you make a show-reel and invite agents.
Yes, some people manage to get through their training in a one year intensive MA and get agents and work at the end but a) they are extremely lucky b) they almost always have many years of previous involvement with the theatre c) they are EXTREMELY hard-working (dd shared a house with a couple of MA students last year and she was in complete awe of them).
A better route in your case would be the 3 year BA-type course. There are also a few non-standard schools running different programmes, e.g. the Fourth Monkey school in London which specialises in physical theatre and currently run a 2-year course.
Since you have no previous experience, you may well find you need a foundation year first to even have a chance to get into the BA (in fact, lots of people need the foundation anyway, just to be able to benefit from the BA).
Even getting into a foundation year at a decent drama school is competitive. Like the standard courses, you have to audition, which usually involves a) preparing monologues in advance b) an interview where you will be assessed for your preparedness in answering searching and sometimes brutal questions about your motivation c) a workshop where the panel will assess how well you take instruction and how well you interact with other people. Auditions can be absolutely gruelling as you are judged not only on skill and hard work but also on personality. Drama schools do weekend and summer courses on audition technique so you might want to sign up for something along those lines.
During drama school interview, they WILL ask you about signs of previous commitment. No, you don't have to have been acting since you were 4, but you do need to show that the theatre is a big part of your life NOW. That all your leisure time is spent on theatre-related things, that you are pro-active and full of initiative when it comes to accessing theatre-related things NOW, and that as for going to the theatre and reading more plays wild horses couldn't stop you (tbh if this is not your mindset, even a week spent with the other drama students will drive you round the bend and drama school would be a miserable place for you)
So if you have that mindset, then regardless of your chances you will be joining an am-dram company this afternoon anyway and spend the rest of the evening reading plays- because you just can't help yourself.