CREATE TABLE table_204_270 (id DECIMAL, "transmitter" TEXT, "kw" DECIMAL, "bbca" DECIMAL, "bbcb" DECIMAL, "d3&4" DECIMAL, "sdn" DECIMAL, "arqa" DECIMAL, "arqb" DECIMAL, "pol." TEXT, "a.g." TEXT)

which is the difference between kw for allenheads and seaham
SELECT ABS((SELECT "kw" FROM table_204_270 WHERE "transmitter" = 'allenheads') - (SELECT "kw" FROM table_204_270 WHERE "transmitter" = 'seaham'))
SELECT ABS((SELECT "kw" FROM "table_204_270" WHERE "transmitter" = 'allenheads') - (SELECT "kw" FROM "table_204_270" WHERE "transmitter" = 'seaham'))