In your shoes (young independent one without family) I would start sailing immediately.
Asking around your local harbour is definitely a good idea. If people see that you want to sail, they be happy to crew you: there are people who feel more comfortable out there with someone else, and finding crew can be sometimes difficult.
If you want to do it seriously, look at www.findacrew.net
. There is a tall ship in Malta (if I can remember correctly) which is recruiting crew for immediate start, and one in Rostock which departs at april. They even pay. There are also various other ships, I just noticed these two. Maybe because I admire tall ships, or maybe because these the ones which seem to be the best ones for the inexperienced.
As you gather miles, you will find ships more and more easily, and learn about sailing. You are English, so you don't really need certificate for small crafts. It would be important if/when you decide to earn a living by delivery, and of course it always help to look at the knowledge you have already gathered in a structured way. But no point in hurrying with it. When you have experience, you can do the tests more easily. Maybe keeping your logbook and making a point of figuring out things for yourself in the old fashioned ways, and using GPS and the skipper's results as a check is a good idea.
If you want to sort of hitchike the world by boat, again findacrew.net is the place to go. The time window now isn't ideal, because the majority of the boats have ealready crossed the pond, but I remember one which is scheduled to start off at February, you can catch it if you are lucky. When you reach the Caribbean you will be a crew with an ocean crossing, and will know if this life is really for you