Jim, thanks.

So the key answer to my questions, and pointers for the OP, a

“For safety anyway, the threshold of the brake activation is set so that going down hill, even if you are in low gear to slow you down, in a correctly set up and maintained over-run system the trailer or caravan’s brakes should not activate… if they did, there is a chance on a long decent in low gear they could overheat.”

I noted with interest this, too:
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“The Future

“As towing vehicles become lighter and trailers and caravans become heavier, there is increased likelihood the towing to the limit of any tow vehicle will become more frequent. Already in Australia and New Zealand, it is now mandatory to have power assisted systems for trailers above certain weights… the same in America and Canada. There are a number of options on the market… AL-KO has had its “Sensabrake” system since 2009 in Australia and New Zealand and in the USA and Canada, there are dozens of companies that offer electronic braking systems.

“In Europe, currently there is the AL-KO ATC system, which is more an electronic stability aid than a full-fledged braking system.”
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Chris N