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It was contrary to regulations. Sir Oswald Mosely was charged under Defense Regulation 18B.
That said, it was possible to express anti-war opinions; lawful opposition existed.
Although the question doesn't ask, @sempaiscuba is correct to point out that expressing opposition wasn't a capital offense; Sir Oswald was interned, but not executed. (I'm not enough of a legal scholar to discuss the boundaries between expressing an opinion contrary to Defense Regulation 18b, Sedition and Treason. As I understand it only the last was a capital offense).