Overly complicated way of "pick a postcode that is a) inside the (multi-)polygon and b) far away from edges"

get_unsplit_postcodes.sql

with
--    n as (select 'Highland' as name),
-- n as (select 'Na h-Eileanan an Iar' as name),
    n as (select 'City of London' as name),
--    n as (select 'Bristol City Council' as name),
buffered as (
	select
		n.name, oo.common_name, f.factor, st_buffer(st_simplify(largest_poly.geom, 0.01), -f.factor) as geom, largest_poly.geom as orig_geom
	from
		n, organisations_organisation oo
		join organisations_organisationgeography og 
			on oo.id = og.organisation_id, 
		-- This next bit gets the largest part of a multipolygon, to make the buffer factor more sane. See Na h-Eileanan an Iar
		lateral (
			select
				(st_dump(og.geography)).geom as geom
			order by
				st_area((st_dump(og.geography)).geom) desc
			limit 1 ) as largest_poly, 
		-- This bit takes a scale of the average of the bounding box edge lengths as a starting point to makeup a factor to buffer by
		lateral (
			select
				0.02*(st_xmax(largest_poly.geom)-st_xmin(largest_poly.geom) + st_ymax(largest_poly.geom)-st_ymin(largest_poly.geom)) as factor
			where
				oo.common_name = n.name) as f
	where
		oo.common_name = n.name 
)
select
	b.common_name,
	b.factor,
	uguo.pcds,
	uguo."location",
	b.geom
--	b.orig_geom
from
	buffered b
join uk_geo_utils_onspd uguo on
	st_within(uguo."location", b.geom)
limit 1