In the 2014 Ringing Trends presentation linked on the Ringing 2030 page has an interesting bar graph demonstrating the decline in the number of Christians in the UK. I have produced an updated version below, with a rough trend for 2031.
In lots of ways this still gives us around 80% of the population who would be open to bellringing as a hobby, because those with no religion are often comfortable to ring (like me). But it does reflect the decreasing church attendance and the risk from there that churches close and become apartments or similar. Although it’s also possible that the core of those identifying as Christian now are the church goers, where many people ticked Christian because they had no active religion but ‘ethnically’ associated with Christianity. So more research to do to understand our potential audience for recruitment!
You are certainly right about the default being Christian in the past. I've actually seen a military form being filled in some decades ago when the conversation went:
Officer: Religion?
Recruit: None
Officer: I'll put down CofE then
Recruit: Well I was born a Methodist but I'm not any more
Officer: OK, Methodist then.