You raise a number of issues in your post.
The first is you age in relation to career plans. If you have just finished 'A' levels you are around 18 or 19? For many people (myself included) aviation is a second career. I got my first airline job aged 36, which still leaves a good 25 years of flying ahead of me. Experience in other industries, maturity and general life-experience are always useful, and can help put flying's pros and cons into perspective. Any job has the potential to become stale over time, but in my (limited) experience, the few colleagues who are jaded and unenthusiastic are generally ones who went straight from school to flying, and have experienced nothing else. It's worth bearing that in mind when listening to people's stories, and also the fact that you have a working life of at least 45 years ahead of you - meaning there's no need to rush into anything. Just as importantly, 45 years is a long time in a boring job.
Which brings us onto the second issue. Parental pressure to follow a particular career is unhelpful, however well-intentioned. Your father may have entirely sensible reasons for suggesting dentistry, but if you have no interest in it, how long do you think it will take before boredom sets in? When it comes to important decisions in life - such as choice of career - by all means seek advice from others, but ultimately you have to do what you believe is right for you. No-one else - even your parents - can tell you what to do.
Regarding investment versus risk, you are quite correct that there are no job guarantees in this game. Add in a self-sponsored type rating and you could spend a lot more than £70k without any guarantee of success. If that though scares you and gives you pause for thought, then good - it should do. I wish more people would stop to consider what they're getting into before committing themselves. If after consideration you feel it's a risk worth taking, then fair enough - that's a calculation that most of us have made at some stage. Remember though that it's possible to get exactly the same qualification for a good £25k less than Oxford or FTE charge, simply by doing modular training - which would also allow you to work full time and pay for training as you go, as well as giving you valuable life and work experience.
Flying is perceived as a glamourous career, and if only for that reason, many, many people have ambitions of being an airline pilot. Few will ever make it. Those who do are generally successful not so much through talent or money, but because they are so passionate about flying that they will put up with all the obstacles and sacrifices on the way to the right hand seat - not to mention the downsides of the job once they get there. Only you know how strongly you feel, and whether it would all be worth it. All I can tell you is that for me, since changing career I have never been happier. If I hear a aircraft overhead I have to look up, on days off I go to airshows. When I go on holiday I miss my job - that's how passionately I feel about flying, and why I can't imagine doing anything else. In short, I'm being paid to do my hobby, and it don't get much better than that. That said, I had the immense good fortune to qualify right at the peak of the job market and sail straight into a job with a decent airline within a couple of months. Not many people are that lucky. Others will have different (and equally valid) opinions, but if you really love flying, then I think you'll find it's worth the risk and the downsides.