Statistically speaking, the Office of National Statistics can't count

As a result of the ONS survey covering a tiny proportion (<20%) of consumption, it is inherently volatile and the BoE has repeatedly said it pays little attention to it, instead prefering to concentrate on their own more detailed survey (the BoE Regional Agents survey) which covers a much larger proportion of retailers and thus shows a higher correlation with the BRC measure.

Also worth noting that in late 2005-2006 the market was very bearish about UK growth prospects and the curve priced in easing, but the BoE measure of retail sales there clearly accelerated whereas the ONS measure was muted. The BoE subsequently tightened policy. In Spring 2008 as the ONS measure was showing strength, the BoE downplayed this, pointing out that both its measure and the BRC measure were showing weakness. There was no tightening.

Anyway, the point is that UK Retail Sales are a lot stronger than the ONS data suggests. The below chart clearly shows that the ONS figures broadly track the BoE survey over time but are much more volatile.

White Line - ONS Retail Sales
Yellow Line - Bank of England Regional Agents Retail survey