Those cabinets are the same as the ones i used to install for NTL now Virgin,
there were objections years ago when they were installed in the "leafy suburbs" never had complaints from the towns.
I worked in Windsor for a time and we actually had protesters sat where we were to install them! Was interesting to say the least.
Usually there are public notifications of intent stuck to lampposts and in the local free rags when street furniture or if any change of access is to be undertaken, often overlooked and if there are no objections then the local council grants permission to proceed.
I don't think for one minute that BT, or any other communications supplier would go ahead at their own discretion and begin installations without prior consent.
They can get taller as they usually install what is known as a "Top hat" to prevent the local riff raff from sitting on them, this adds about another 500mm to the top and is more peaked.
The foundations for these can be quite substantial, designed for vehicle impacts etc. but when i installed them, they were never allowed to infringe on minimum distances and restrict public access be it wheelchairs or pushchairs/prams. ( if memory serves me right I think its 1.2m width )