Large National Grid substations do not produce significant electric fields outside their boundary because the perimeter fence screens the electric field from any sources within the substation.
Inside the substation, the various busbars and other equipment do produce electric fields. These are highest where the arrangement of busbars means that the fields from adjacent busbars reinforce each other, and lower where the arrangement produces cancellation.
This diagram was produced in the 1980s after extensive measurements in a 400 kV substation chosen because the arrangement of busbars was likely to produce the highest field possible.
It is not very clear to see (download a hi-resolution version if you want to see all the numbers), but in most regions, the field is between 5 and 10 kV/m. But in small regions where the busbars reinforce each other, the field rose to 21 kV/m:
Note that these readings were taken at head height, 1.8 m above ground. The standard now for assessing fields is 1 m above ground, where the fields would be slightly lower.
See also contour plots of magnetic fields in a substation.
