In SE1, Morley College runs beginners' ballet courses, but having never taken them I can't recommend them personally. I did take a similar course at the City Lit (Aldwych) a couple of years ago, which I thought was very methodical and educational.
I took intermediate ballet lessons (I don't remember which level - perhaps 3) at Morley some years ago & I found that the quality of the teaching was quite high. I had trained to RAD Elementary level as a teenager & the class was quite suitable & challenging after a 10 year break. I'm not sure which syllabus they followed but fundamentally all the movements you learn are similar regardless of approach.
I think there were some girls in the class who had come through from beginner classes & they seemed to have a good knowledge & technique so I'm sure you will find the classes there a great introduction.