I'm not so sure it'll matter that much.
At least where I went to undergrad, the social work degrees started at the Master's level (for 'child protection' - I know a guy who used to be a social worker and now directs small independent horror movies - so background might potentially be more fluid than you might think). As for teaching - study what you would like to teach/what you can get the best grades in. Also where I went for undergrad, there was a college just for education and future teachers went there, learning things like child development, etc.
I also know teachers in the US (I realize you are in the UK, probably, as the term 'uni' isn't really used in the US by most people) who didn't take any sort of education courses but instead just studied for and passed the licensing exam.
One practical thing you can do is to start considering schools you might like to attend, based on everything from cost to location to reputation, etc., and see what their requirements are. A generalized list of requirements won't help you that much if your dream school turns out to have a different plan.