Since early March 2020, the COVID-19 epidemic across the United Kingdom has led to a range of social
distancing policies, which have resulted in reduced mobility across different regions. Crowd level data
on mobile phone usage can be used as a proxy for actual population mobility patterns and provide a
way of quantifying the impact of social distancing measures on changes in mobility. Here, we use two
mobile phone-based datasets (anonymised and aggregated crowd level data from O2 and from the
Facebook app on mobile phones) to assess changes in average mobility, both overall and broken down
into high and low population density areas, and changes in the distribution of journey lengths. We
show that there was a substantial overall reduction in mobility with the most rapid decline on the 24th
March 2020, the day after the Prime Minister’s announcement of an enforced lockdown. The
reduction in mobility was highly synchronized across the UK. Although mobility has remained low since
26th March 2020, we detect a gradual increase since that time. We also show that the two different
datasets produce similar trends, albeit with some location-specific differences. We see slightly larger
reductions in average mobility in high-density areas than in low-density areas, with greater variation
in mobility in the high-density areas: some high-density areas eliminated almost all mobility. We are
only able to observe populations living in locations where sufficient number of people use Facebook
or a device connected to the relevant provider’s network such that no individual is identifiable. These
analyses form a baseline with which to monitor changes in behaviour in the UK as social distancing is
eased.