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:
DO I NEED TO APPLY FOR ESTA IF...?
If I have 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.
Source: Frequently asked questions.
No, you do not need to apply for ESTA as your visa supersedes VWP eligible travel.
ESTA is for travel under the VWP, the visa-free arragements applicable to Brits. As such, as you're not travelling under the VWP, you don't need an ESTA.