Anywhere with third rail (which is predominantly London and the South-East of England) tends to be fenced off along the sides of the tracks or other things in place to strongly discourage you from walking onto the tracks.

Given that a considerable amount of the UK rail routes date from the late 1800s there are a lot of places where tracks cross roads and therefore mix with other forms of transport (including pedestrians). It's surprising just how little there is in between a pedestrian and a live rail in these situations, here's an example 10 miles or so away from central London: https://maps.app.goo.gl/nPcJM1YxBexaDDKY6

One of those live third rails start less than 5 yards away from where pedestrians regularly walk, with just some angled planks of wood to stop you walking towards them.