In the UK braked (i.e. pretty much all) trailers and caravans must have a break away cable that applies the hand brake if the trailer coupling comes off the ball. The cable is attached to a point on the towbar by a clip that is strong enough to apply the hand brake but weak enough to break away allowing the vehicle and trailer to separate and leaving the trailer to stop on its own.

I wouldn't fancy being followed by an uncoupled unbraked trailer attached by a strong chain

I think this is matter of opinion. And my opinion is based on trailer actually coming off ball and the chains allowing the trailer tongue to rest on chains. In my case, applying the brake had trailer tongue going under the hitch and the trailer hitting the back bumper. This was not a glider trailer but a utility trailer. Light weight trailers are not required to have a brake but are required to have safety chains. Redundancy is always better in my opinion and I would rather for chains to keep tongue from catching and catapulting or piercing something as first safety than have a braking system to take effect if the chains fail. I would think chains failing would be a less likelihood than a braking system failing. Chains have very little maintenance while the small cable, braking system, and actual brakes are a little more complex and require all to work properly.