Within the UK, there are areas with a relatively high population density like south-east England, and areas of low population density, like north-east Scotland.

The litter density was almost 60% higher in coastal areas compared to inland regions ... Citizen science reveals litter trends in the UK: Population density effects on coastal and inland regions ...

Population density (often abbreviated PD) is a compound measure that tells us roughly how many people live in an area of known size. It is commonly used to compare how 'built-up' two areas are.