I am British with a B1/B2 visa for USA. Do I still need an ESTA?
No. Here's what the US government has to say on the question (see this answer's edit history for the version that was available when the question was asked):
What if a traveler has a current, valid visa?
Individuals who possess a valid visa will still be able to travel to the United States on that visa for the purpose for which it was issued. Individuals traveling on valid visas are not required to apply for an ESTA authorization.
I have a B1/B2 visa. Do I need to apply for an ESTA?
If you already have a B1/B2 or any other valid visa and you are traveling for the purpose in which it was issued, you do not need to apply for an ESTA authorization.
Source: Frequently asked questions.