Do I need a transit visa for uk when travelling from Zimbabwe to Los Angeles by air, layover less than 24 hours in London
The Gov.UK Official Website has a great tool to help you out in these situations. Feeding your information on the tool and assuming that you are a citizen of Zimbabwe, or for that matter, in this case most countries you won't need a transit visa if you already have a valid US Visa.
Since you are traveling to Los Angeles, that would imply that you have a valid US visa and hence you have no need for a transit visa.
Note: I assume that you will not be leaving the airport during the layover and both your arriving and departing flight are from the same airport i.e. you should not be passing thru UK Border Control.