Good question. There are probably quite good at measuring well. However, Considering that they say the fall is mostly due to the energy cap, the better question might be...Are they measuring the right prices? There's no right answer, I suppose.
The ONS has multiple values depending on which selection of prices you're interested in. EMG. With/without fuel, and one assuming a standard shopping basket which they regularly update to make sure it's what people are currently buying.