Turns out my issue was not really related to encoding at all. The human-readable version of the geometry in a PostGIS-like database will only be returned when ST_AsText is used in the select statement. So my SQL query becomes:
SELECT inza..ST_AsText(t.SHAPE)
FROM (SELECT * FROM OS_AB_PLUS..E12_ADDRESSBASE WHERE POSTCODE = 'RH1 6NE'
AND PAO_START_NUMBER = '14') as a, OS_TOPO..TOPOGRAPHICAREA as t
WHERE inza..ST_Within(a.shape, t.shape) = TRUE
Which returns. as intended:
POLYGON ((526696.15 148931.9, 526703.94 148932.34, 526703.8 148935.2, 526705.5 148935.3, 526705.4 148937.8, 526695.9 148937.35, 526696.15 148931.9))