I don’t know about building codes in Britain, but in almost all of the US it would be illegal to build a carbon copy of the beloved older cities we have, because huge roads, parking lots, and setbacks are all mandatory.
In North America there are planners and activists trying to change this[1] & world-wide, planners can achieve walkable, people-centric urban planning [2].
[1] - https://www.strongtowns.org
[2] - https://www.youtube.com/channel/UC0intLFzLaudFG-xAvUEO-A