Flat heads are used on most recent bells. Bells earlier than around 1910 were not able to cast bells without canons because they didn't have drills strong enough to get through the bell where as today they do!
In Previous times they would have either made a mould with the canon on the bell or they would have made the bell and canon separately and then welded them together.
You will see the majority if not all of the bells without a canon are hung on an iron headstock.
The bell is connected to the headstock by nuts and bolts which go through the bell.
If you see old bells that don't appear to possess a canon, it is likely the canon has been removed.
The benefits of having a flat head is that a hole for the clapper can be drilled which reduces the risk of the bell cracking due to an older style of crown staple.
As bell founders realized they could drill through bell metal, they realized that they could drill through the crown and therefore a canon was no longer necessary.
A Sussex example of bells that have Flat Heads are Worth, St Nicholas.
I have not added a sketch of the canon as there isn't one present on a flat headed bell.
Images of bells with the canon:
Coleman's Hatch Holy Trinity; Ashdown Park, Convent of Notre Dame; John Taylor & Co Bell Foundry; John Taylor & Co Bell Foundry.