You could perhaps rephrase that as "the places where most people live in the UK aren't hilly". Much of the middle of Wales is hilly, as is much of rural Scotland and northern England, as is that fairly large area of nothingness west of Leicester and east of Birmingham, north of Northampton and south of Rugby, but comparatively few people live in those places.
But this is just being finiticky as
@A0wen was. The points are twofold:
1. A
lot of people in the UK live in places where cycling with an unpowered bicycle is entirely feasible for any able bodied adult.
2. For those who don't, an e-bike will remove the hills for you.