> I'm not a Christian but I'm pretty sure I'd still need to deconsecrate a church before knocking it down

What makes you think that? If you own it, it isn't a listed building, and zoning laws allow it, you can knock it down in most countries. I'm not sure that would be illegal anywhere.

Deconsecration is part of canon law for some religions. Whether you as an individual care about those or not is a personal matter. I doubt that there are many cases where the previous owner forgot to deconsecrate the church when they sold it off.

UK law can be quite intertwined with the church in a lot of cases, so I wouldn't want to confidently say this wouldn't turn up somehow. And churches are also pretty likely to be listed, anyhow. (It could also turn up as part of the planning permission which you do need for demolishing a large structure in the UK)