I recently applied for the US B2 visa and have an interview coming up. However I have now realised how difficult it is to be accepted for this visa as a U.K. citizen. I want to cancel the interview which I know is possible, but will cancelling and not rescheduling an appointment affect me using the ESTA waiver programme in future as I know I am eligible for this? I know illl lose the visa fee I paid but that isn't a problem
Grounds for ineligibility for the visa waiver program include having been denied a visa. If you withdraw your application, you cannot be denied a visa, so you will preserve your eligibility (assuming you aren't ineligible for some other reason).
It might be a good idea to withdraw your application formally rather than just to cancel your interview and not reschedule.
You should also keep in mind that if you visit using the VWP you will of course be subject to the slightly more restrictive terms of that program. For example, your stay will be limited to 90 days.