If you want my advice, do a basic Japanese course in your home country and then go and live deep in the Japanese countryside where not a soul speaks English and the only way to even begin to get to know anyone is to pick up the language. It'd proabably cost a fraction of the cost of attending a language school. You'll be amazed how much quicker you learn. Some of the best Japanese speakers I have met, did exactly this. It's quite possible to live in Tokyo for years and hardly pick up a word of Japanese. It doesn't just soak through your skin by osmosis. You need a reason to learn.
To 'get by in daily life' is a misleading concept. Most of the things people do every day (shopping for food, taking public transport, going out for a beer) actually require almost no language skills at all. I don't know how old you are, or how much you depend on other people, but your two other questions (is Japan dangerous, are there many English speakers) are answered elsewhere. But briefly, 1. Japan is considerably safer than the UK so you'd be moving to a place with a lot less crime. 2. Yes, it's not hard to find plenty of people who speak English (both Japanese and foreign) but to reiterate my first comments, the less English speakers you mix with, the better. Otherwise you'll very quickly find yourself slipping into the gaijin bubble, and never really gaining the benefit of living in Japan.
Another point to bear in mind is: have you checked out the language school running the course? There are some really great schools and some lousy ones. You could learn double the amount of Japanese in the right class with the right teacher than if you end up at a lame school.