People pushing big prams are never guilty of blocking pavements to have an impromptu group chat, of course...hmm
So you assume most people with prams travel in groups and block the pavement? I am always careful not to do this. That's rather like saying everyone with a wheelie-suitcase walks carelessly and trips people up with it.
Yes, of course people who do not have prams are guilty of blocking others too BUT they are not holding a biggish piece of machinery which might hurt someone's ankles or legs. That is the point
So it's ok for those without prams to block others? 
A pram is hardly a 'piece of machinery'. Mine is small and light. Are you saying people with prams must always walk very slowly, in case someone unexpectedly stops/side-steps/walks into their path? Isn't it everyone's responsibility to watch where they're going? What do you think about people with shopping trollies, mobility aids, wheelchairs, suitcases on wheels, must they be restricted to slow speeds too? All of these 'biggish pieces of machinery' could hurt someone's legs or ankles, and can be cumbersome and heavy to manoeuvre... so why not do everyone a favour and give them room to pass? If they don't need to dodge around you, your ankles are safe.
If you must amble slowly down the very centre of the pavement, stand chatting in doorways, walk 3-abreast or text/smoke/daydream as you walk, at least be aware of others stuck behind you and let them pass!
Everyone has the right to get from A to B at a speed that suits them, and a responsibility not to collide with or obstruct others.