I'm taking a gap year after uni - I didn't want one before starting uni. It just didn't appeal to me... I was worked so hard to get into uni and got on to the course I'd worked through out 6th form for. I think I just kind of wanted the momentum to continue!! I guess I should also mention that I have a job lined up for when I get back. I see that you want to be a teacher - I guess that it would be possible for you to defer your work start date too? So, anyway, my experience might be a bit irrelevant to you but I figure I'll provide the 'other side' and talk about what I've found to be advantages of taking a gap year after uni.
Firstly, coming out of university you're (obviously) more experienced than you are at 18 so you can earn much more as a temp. This summer/autumn I've been working for a law firm (I did a law degree) and got about £18 a hour for what was essentially an admin job. They wouldn't have considered me if I wasn't degree educated. It hasn't taken me that long to save up for my trip. If I was 18 and waiting tables on min wage it would have taken much longer.
Secondly, the earning potential thing means that you just have more money available to you and that gives you a greater choice in what you do - I decided that I didn't want to do one of STA's round-the-world routes, picked several countries that I have always wanted to go to and could afford to do so. I'm going for longer and further than I would probably be able to do otherwise.
Thirdly, being that bit older means I can do things like drink in the US and hire a car.
Fourthly, especially if you've been to uni you are probably less worried about doing exactly what you want, where you want - I'm going to NYC by myself. Other friends of mine that have done a post uni gap year have gone places by themselves like Toyoko, Miami etc that are not traditional gap year destinations. Obviously there are going to be people here who've done this at 18 but in my experience, not as many as who've done it in their early 20s.
I guess my experience is a little specialist though. I wouldn't say that doing a gap year after uni is better than doing one after, its a totally different experience.. There's no right or wrong decision - its simply what suits you and your finances at the time.
I would also say that I didn't find going straight into uni after school a difficult experience. Most people don't have a gap year, you're certainly not going to be the only one. In fact, you'll be in the majority if you don't have one. Although that's not a reason not to have a gap year, its not like if you get there and haven't had one, you're suddenly going to be surrounded by people who have spent the last year enriching their soul whilst you slogged over your textbooks. Additionally, you remember what you learnt at A-levels: if you're doing a subject that relates to your previous study this can make your life a lot easier! Whether you feel that you need your confidence built / experience widened etc by a gap year is something that only you can decide. I think that this is a definate advantage for some people, but its very much an individual thing. I changed schools a lot so I wasn't particularly daunted by the prospect of uni and I don't think that I was particularly immature when I started uni compared to the people who had had gap years.