Head.

This widely used example of Cockney rhyming slang is said by Eric Partridge in his Dictionary of Slang and Unconventional English to be late 19th century. I can't find examples of it in print from then though - the earliest I've come across being the definition in Fraser & Gibbons' Soldier and sailor words and phrases, 1925:

"Loaf, head, e.g., 'Duck your loaf - i.e., keep your head below the parapet'."

The common phrase 'use your loaf' clearly derives from the rhyming slang and an example of that dates from a few years later - James Curtis's novel They drive by night, 1938:

"Bloody seconds counted in a job like this. You certainly had to use your loaf."

This was defined by Hunt & Pringle in their Service Slang, 1943:

"Use your loaf is the injunction often heard when someone is particularly slow in following orders. But this phrase, in its finer meanings, says: 'Use your common sense. Interpret orders according to the situation as you find it, and don't follow the book of words too literally.'"

Other phrases first cited in Fraser and Gibbons: