tow bar electrics

Geoff.  Modern cars are complex beasts.  Go for the vehicle specific loom.  This will ensure that all your car gizmos continue to work, or cut off as necessary, when your car/caravan electrics are connected; you should also get an indication when there is a light failure on the caravan road lights (eg indicators a legal requirement for some sort of indication if not working).  I must say I am a little surprised that the option is still available.

To comply with the law, it's good enough that indicators rapid flash, like your car, if one of them fails ... ie you do not need a tell tale lamp inside the car that flashes in time with the caravan's indicators.

Go for a vehicle specific loom.

Caravans that have LED rear direction indicator lights do not draw enough power to signify on the dashboard that they are working, so if they fail by way of a poor connection or diode failure you will not know. Hence you need either a unit known as a VLM or some sort of load that will draw power to make the onboard dash lamp to flash at the same rate as the indicators.

With out the above unit fitted the car will not know that a caravan is connected.

If you have led indicators &/or tail lights & one fails, there's little you'd be able to do about it till you get to the site or back home.

I'll ring a fault on my trailer board & see if my car's factory towbar bar wiring notices. 👍 There's no extra tell tale on the dash for indicators unlike my older Vauxhalls had.

4.9.1. Presence, condition and operation
You must inspect the following lighting ‘tell-tales’:

headlamp main beam
direction indicators
hazard warning lamps
rear fog lamps
A main beam ‘tell-tale’ is only required on vehicles first used on or after 1 April 1986. Class 3 vehicles do not need to have the main-beam tell-tale inspected.

A direction indicator ‘tell-tale’ can be audible or visual.

A hazard warning ‘tell-tale’ must be a flashing light.

A rear fog ‘tell-tale’ is only required where a rear fog lamp is mandatory.

Defect
Category
(a) A mandatory tell-tale:

(i) for direction indicators or hazard warning missing or inoperative
(ii) for main beam headlamp or rear fog lamp missing or inoperative

Minor
Major

The above taken off the Gov’t web site

as you posted ... an indicator tell tale can be visual. If you can visualise it flashing rapidly, then it's complying with the law. 

Yes but with LED trailer indicators they do not draw enough power to warn the driver that they are operating, that is why a VLM unit or other means of increasing the power to validate that the trailer indicators are working