Ok, since the polygon seems to be inverted, here is the explanation:
Each polygon on a sphere has complimentary one that occupies everything else on the planet. They share the same boundary, and are described by exactly same vertices. Say, oceans and continents are described by exactly the same coastal line. How do geospatial systems distinguish between them?...
To change the type of a column, you need to use ALTER TABLE. Use NULLIF to convert empty strings into NULLs:
ALTER TABLE MyTable
ALTER COLUMN geom
SET DATA TYPE geometry(geometry, 27700) -- or geometry(point, 27700)
USING ST_SetSRID(ST_GeomFromText(NULLIF(geom, '')), 27700);