Not having to tell anyone is a major part of HMR&C's reasoning behind the
2,500l threshold. They were getting bogged down by people registering for tiny amounts of production that were not worth the staff costs to administer and collect the duty.Even though you don't have to tell anyone below 2,500/l per rolling 12 month period(*) it would still be very advisable to keep evidence that you are below the threshold.
(*) Fairly sure it's a rolling 12 month period rather than a fixed calander year. Dig out a recent post of mine with links the the HMR&C site.