I can tell you, with some experience of the police force, that a traffic cop will be on the lookout for anything unsafe.
If it doesn't look unsafe, he won't be interested and he certainly won't be familiar with any regulations - other than a general knowledge of light positioning and weight ratios.
Therefore, you can build a trailer and use it will impunity as long as it looks to be safe and in good condition (and hopefully it is!)
That pretty much sums up the situation in general. But ...
The most likely reason for stopping a vehicle is a lighting fault so make darn sure you use a double-bulb system and/or LED lights. The other reason is a load that looks unsafe or too heavy or faulty tyres.
I've been pulled up with a completely roadworthy and legal setup, and one not at all looking "dodgy". As it happens I knew the copper (fellow member of the local Land Rover club), but he started with "that's the sort of thing we're keeping an eye out for" (or something like that). But as you say, once he'd seen there was a breakaway cable and the lights were plugged in, that was it. After that we had a chat about Land Rover and stuff.
But the big issue to consider is not being pulled during a routine journey. It's what happens if "something" happens.
Over in the land Rover forums, there are lots of discussions about various things that may or may not be legal (fitting LED bulbs in standard holders, or fitting non-E-marked lights, for example). Again, the consensus is that you won't get stopped for it, and few would know the regs inside out anyway.
But find yourself involved in an accident and it's a different mater. You may be a completely innocent party, but the coppers are likely to have a good look over your vehicle. All it needs is a "bright young thing" out to impress the boss and your creative interpretation of the regs could just be one of those "bingo" charges - just something else to charge you with in case nothing else sticks.
Also I occasionally see roadside checks where the Police and VOSA are there. They're mostly looking for dodgy commercial vehicles or traders daring to carry an empty cardboard box without a waste carriers licence, but I've also read of them specifically targeting caravans and trailers (caravans have a poor track record !) - and you can expect a VOSA inspector to know the regs fairly well.