The OU is a fantastic institution (and I don't just say that because I have an MA degree from it). It was set up over 40 years ago to provide the opportunity of getting a degree for people who might be good enough to do it but just didn't fit in the academic system as it was then. It has a policy of no entry requirements whatsoever - if you think you can do it, enrol for a level 1 course and see how you get on.
It is entirely about "distance learning" and doing it at home. Milton Keynes is just the HQ where it is all run from. You enrol, they send you the materials through the post and tell you what other books you should buy and read, you work through it and send your assignments (TMAs) to your tutor, and they mark them and send them back with comments - usually quite a lot of comments to help you learn. You also have contact details for your tutor so you can ask them something if you really get stuck. Tutors are quite often university teachers at other universities doing this to earn some more money. Different courses use all kinds of methods to suit. Popular courses can include tutorials where you can go along and discuss things with other students, though it isn't compulsory to go to them. On my MA, we had occasional online tutorials where over a weekend we would have online discussions like sharing on a message board - dip in and out when you have the time and comment on what each other is saying, the next best thing to a proper university tutorial class. Some courses have summer schools where you go to a "real" university for a week. And after 40 years of experience, the OU is the best at all of this. Most courses have a formal written exam at the end, usually counting for half the marks, so that is the time you actually do need to take the day off to go to where it is held, and it'll be just like sitting GCSEs at school. My first degree was full-time at another university so I appreciate the differences and how good the OU really is.
Every course has a points value. For a BA or BSc, you need to accumulate 360 points. Most courses lasting a year are 60 points, so if you're doing it while in full-time work the normal thing is to do one a year for 6 years. But if you think you're up to it and have the time, you can do two at a time and finish in the usual 3 years that you would if you went to another university. The usual problem people have is the self-discipline you need to keep going when you don't have any actual classes to go to. This is one reason for regular TMAs - having to send in work regularly forces you to keep going. Sesame ("Open Sesame", geddit!), the student magazine, sometimes contains letters from OU students asking why the OU can't be more flexible about TMA deadlines. The reply is always what I've just said - the whole point is to keep you on track. If you find that doing two courses at a time is getting too much for you, fine - just choose one next year and it will take longer to finish the degree.
I warn you, once you've tried a course and found you like it, it can get quite addictive! I heard some of that at my graduation ceremony - yes, you do get a proper graduation ceremony with gown, hood and everything if you want to go.
The way to qualify as a teacher is to get a degree then do a PGCE. Years ago there were four-year BEd degrees where you could do the two combined but those seem to have disappeared. The OU offers a PGCE too http://www3.open.ac.uk/study/postgraduate/qualification/k20.htm but currently that is only available in six secondary school subjects. For primary teaching, you'll have to go to a conventional university and that will be one year full-time. As the PGCE has to include long periods of teaching practice anyway, that may not make much difference to you.
The OU has an extensive web site, as you will have noticed, and you can choose between a number of undergraduate degrees or even go for the Open degree, where you do six courses in whatever you like. (Originally, that's the only one it offered.) There are a number of BA degrees relating to primary education http://www3.open.ac.uk/study/undergraduate/qualification/education/index.htm - click through to see what six courses they require you to do, or the same with any other degree that takes your fancy. Every course has a page describing exactly what it entails. For primary teaching it doesn't matter much what you do.
Just to provide an example of a course, a very popular starter if you're more an arts/humanities person than a science/maths person is this one http://www3.open.ac.uk/study/undergraduate/course/aa100.htm