The last unmarried king of England was King Edward VIII.
Details Regarding Edward VIII
Edward VIII reigned for a very short period, less than a year, in 1936. His unmarried status at the time he ascended the throne is what makes him the answer to the question. He ultimately abdicated the throne in December 1936 in order to marry Wallis Simpson, an American divorcee. The Church of England, of which the monarch is the Supreme Governor, did not then permit the marriage of divorced persons if their former spouse was still living.
Important Distinction
It's important to note that while Queen Victoria was unmarried when she became queen in 1837, she subsequently married Prince Albert in 1840. Edward VIII remained unmarried throughout his brief reign.
