If you have the ability and interest I would wholeheartely recommend going to Uni and getting a degree before flight training.
Yes you could working in a supermarket for those three years (at 15K per year), but you'll die of boredom after the first one. Yes a degree 'takes' three years of your money-earning life but (i) You can work part-time during university and full time during the holidays to keep the PPL going (ii) You'll probably not be stacking shelves and earn on average 25K asfter leaving:
http://www.prospects.ac.uk/cms/ShowPage/Home_page/How_much_could_I_earn_/Salary_and_vacancy_FAQs__2_/What_is_the_average_graduate_starting_salary_/p!eaLXbeX
Plus you'll get a qualification that you can pull out for a back up career if something goes pear shaped along the line. Of course also there are the intangibles that have been mentioned in previous posts.
From my own personal experience, I thought I was grown up enough at 18 to give uni a miss and start working towards flight training. To cut a long story short, things got messy at home and I ended up going to uni after a year in work. After graduating I looked back on my experience to realize that at 18 I still had a great deal of maturing to do. I would have to say that my undergrad days were best days of my life (so far!) and I wouldn't take those years back for anything. At 18, you are mature enough to know the direction you want to go and are very driven, but because you are 18, you really haven't seen much of 'life' (I sound like an old man!). Three years at uni are really a good boot camp and the experience will help you both at the study and interview stage for a flying job.
Although most people say "do a subject you like" I would urge caution and get degree that you are interested in, but has a useful application afterwards. Don't do a flimsy 'hobby' degree (e.g. 'Rock music' or something), bit the bullet get yourself a firm technical degree (engineering, biotech etc etc). I say this because the people I know who did those mickey mouse degrees had fun and grew up over three years but are now back in the supermarket at 15K.
Just my opinion based on my personal experience!
Good luck with your choices,
Gareth.