There are a number of concerns here, so I'll deal with them each in turn. I originally went to college for Philosophy, but dropped it in favor of computers and psychology! So you might say I'm heading the opposite direction of what you are thinking, so I hope I can provide you some things to consider.
The first concern is how familiar you are with the specific work of modern, living, publishing philosophers? While it is technically true that philosophy questions everything, that doesn't mean that everything is an open question today - countless lines of inquiry have been beaten to death over the last 2000+ years. This does not mean it's not still a line of study - in fact, one of two lines of specialty offered at Oxford is indeed Ancient Philosophy (Aristotle, Plato, etc). It seemed a lot more like history and classic literature to me, which I was not expecting and had already ruled out as interests myself - but if you love it, then great! But that's not at all what I thought I was signing up for, and thus the importance of fact-checking reality before making a decision.
The second concern is about writing something to get into a Masters/PhD. Actually, you aren't far off the mark here, but be aware that what most departments are interested in for admission isn't a book or a thesis - it's a writing sample! I've often heard that these are expected to range from 10-20 pages at most, but every department is different so you'll need to check.
The third concern is attempting to go directly into philosophy without significant undergraduate study, but you are targeting top-in-the-world programs. Check out Philosophy Graduate Admissions at Oxford statements:
BPhil in Philosophy
The BPhil is an intellectually demanding course, presupposing an
undergraduate and/or graduate background in philosophy (or
equivalent). It is not suitable as a conversion course for students
changing to philosophy from another subject and it cannot be studied
part-time or externally...
MSt in Ancient Philosophy
The MSt course in Ancient Philosophy aims to attract students of the
highest calibre - with a background in philosophy or
classics...Knowledge of ancient Greek language is not a prerequisite
for admission to the course. However, students admitted will be
expected to achieve Intermediate Level ancient Greek...
MSt in Philosophy of Physics
This course aims to attract students with a strong background in
physics at undergraduate level or higher...
So if you don't want to question specifically ancient Greek or Physics, and if you aren't willing to start back again at undergraduate courses in philosophy, you are looking at the wrong department - they just don't seem interested.
How about UCL Philosophy (under Entry tab):
Entry requirements
A minimum of an upper second-class Bachelor's
degree in Philosophy from a UK university or an overseas qualification
of an equivalent standard. Applicants with a strong degree in a
non-philosophical subject are welcome.
Aha! So that last sentence isn't a no, at least!
I believe you'll find this general trend is true - if you check with the specific departments, you'll get a much better feel for who will even consider admitting you under normal circumstances. However, that leads into...
The fourth concern is being aware of the relative competitiveness you're looking into. As noted in various blogs and directly from some departments websites (Oxford lists some info on theirs linked previously), Philosophy is one of many fields where interested people outnumber positions available. 10:1 to 100:1 ratios of applicants to available positions are often reported - though be aware that this includes international students. If you are looking at UK institutions and you are already a UK citizen, or otherwise looking into places in your own home country, the odds are vastly better than quoted - but there are still usually many more applicants than positions available.
As such, especially if you want more in-demand departments, you'll be at a comparative disadvantage if your interests in philosophy aren't linked to your previous degree, or if you don't have a degree in philosophy or at least lots of coursework. Some programs will exclude you entirely, and many more will not accept you into a PhD directly and will require a Master first at least (with many PhD programs not really having much coursework, so if you don't have that at an undergraduate level they aren't prepared to provide that as part of the program).
This is not to say that you can't seek a career in philosophy if that's what you decide to do - but writing an interesting book or paper of a general philosophical nature won't get you right into a PhD anywhere of note. You might very well be able to get into a Masters program if you can show the program what they are looking for, but you'll need to locate a program that seems like a good fit and tailor your application to them specifically. So I'm by no means telling you it isn't possible! But I am saying it will take a lot of hard work to find the right fit, and make sure it's right for you to start with.
I hope this can be food for thought, and regardless of what you find out - good luck! As a parting piece of wisdom, I would advise this - keep your mind and eyes open. There's lots of fields and lots of really intersections of interests and passions - you never know what you'll find on the way to your dreams!
