It’s not as simple as “is it hilly or not” (and I spent many years cycling in Bristol). The key factor is more likely to be “what proportion of the journeys I make are steep hills / dangerous roads etc).Correlation is not causation. Dutch cities like Maastricht and Arnhem are just as hilly as many UK towns and cities. Likewise Bristol is very hilly but more folk there cycle because there has been some investment in safe cycle lanes. Not enough IMO, but enough to begin to make a bit of a difference. The key point is you have to make it safe to attract the masses.
As an example, the viability / desirability of non-assisted cycling in my now home-city of Bath will be determined by whether your journey is principally East - West or principally North - South. Of course hills matter.