So far I have not found a definitive/reliable explanation.
The most plausible is that it is derived from a noble man who had lands on the Devon/Cornwall border. There is a still a small hamlet by the name of Brenton in Devon.
Another suggestion is that it is a corruption of Breton, with the Brentons origionally coming from France - I have not got any proof of this in any research that has come my way - YET!
By the way are you able to connect your William Brenton back beyond 1600 and Hammersmith, London?