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

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'))