Women step back too, or at least they ought to...
London tango seems to me to be between eras. Generally, people learned and still learn to dance in 'open embrace' (which isn't an embrace at all!) That's inevitable at present. In open embrace you're in contact with your partner with your hands and arms so it simply doesn't matter how you walk. But when you embrace your partner, torso to torso, the whole dance changes. How you walk suddenly becomes important. Perhaps teaching here hasn't caught up with this change in the kind of dancing.
This becomes particularly obvious when I dance with a partner I've not met before; I step forwards and my knees bump her knees. Oh no... She assures me that in her beginners' classes there's a lot of walking, but I suspect it's an emphasis on walking to the beat, rather than on posture and the mechanics of walking suited to close embrace. My partner is walking backwards as you would in normal life: her knees come up a bit, and then as each foot goes down her torso jerks slightly backwards. Which is fine in normal life, but it's a dangerous combination to anyone dancing close with her. Maybe she's been told and it simply hasn't registered that it's important, or maybe walking just hasn't been taught in the classes she's been to.
(I remember the story Christine Denniston tells in a short film about tango: she was taught to walk at her first class, and went home and practiced it every evening for a month. It was years ago now, and she didn't mention who taught her, but she practiced it to perfection: when she went to Buenos Aires she says she fitted in easily as a dancer.)
It's simple enough to step back in tango. The woman reaches back with her foot, to some extent straightening her leg. Her other leg, the leg her weight is on, might flex a bit, which can give energy to the step. It's not stepping back in the everyday sense, it's reaching back. Well done, it looks great, energetic and purposeful. Reaching back has a second effect: as you reach back, your torso pushes forwards, which means the embrace is firmer: perhaps this is how the really close embrace of the Buenos Aires dance arises. The pivotal point is the lower back, and perhaps that's why this aspect of tango gets ignored here. If your lower back is weak, 'reaching back' might feel uncomfortable at first. & if you are hesitant about committing to close embrace you might not want to push your torso forwards.
In Buenos Aires it's taken for granted that tango is danced close, and even complete beginners are expected to dance close. I've been to all the group classes and pre-milonga classes I could, and in all of them walking can take up the first 30 or 40 minutes of a 90-minute class. It's walking to the beat, and also correction of posture and the practice of walking. Cacho Dante gets his assistants to take a separate class for newcomers, where they only walk. He's strict about it; until their walking is good, they don't join the main class. Some very beautiful dancers come out of his classes, dancers who look at ease, effortless and comfortable even in crowded milongas, as they've been well drilled, from the basics of walk upwards. Sadly, I've never spent long enough there to become that well drilled. & I think Cacho allowed me into his main class out of politeness: I suspect he really thought I needed a month or so with his assistants, practicing just walking.