David the triangle sticker does not meet the rules in all respects, shape, size colour and backing. I believe you can buy rectangular yellow with red border (and no chevrons or other big lettering) in Uk - I bought mine at vast expense in Spain a couple of years ago when the 'revised requirement' was first mooted.
The degree of the check is unpredictable as it depends entirely on the individual GC officer/s making the check ( and possibly how you respond to them). They are, after all, only human, and often under quite a lot of pressure to meet targets - just like UK.
I have never (yet) been stopped for such a check, but have reliable information that measuring tapes have been used at places like toll booths. I gather that plates of UK supply may not quite meet the dimensional requirements but only by a few mm. either way.
I have checked my overall outfit length carefully and it is sufficiently above 12m. to make it not worth the risk. The documentation will show shipping length of the caravan and the normal length of the car - the bit missing will be the length of the drawbar less the ball.
In practice it is quite difficult to line a tape up 'square' to get a decently accurate reading say to plus/minus 1 cm as most cars have slightly curved front bumpers and which bit of the caravan rear do you measure to ?
All in all, if, like us, you are looking for a relaxing, trouble free holiday, then i suggest it is worth going the extra mile / kilometer to minimise the hassle factor. For the same reason we have the mandatory day-glow vests rooed up and secured to the rear of the headrests so they can be seen from outside the car, the bulb kit and spare specs. easlity to hand and so on. Some would say this is defeatist and we should stick to our 'rights' - many of our dutch friends said exactly this - if it's legal in Holland it's legal in Spain but went a bit quiet when I asked why, on this basis, I was expected to deflect my dipped beams when i visted their country.
I confess that my attitude is influenced by my only brush with GC in Spain which cost me 105 euro as my wife had not yet fastened her seat belt when a few yards from the camp exit, but I cannot argue that we had broken the law. The penalty seems excessive as the only likely victim was the perpetrator, but there you are. In the same area the GC were very active in the restaurent area after dark, often breathalysing all occupants of the car rathe than just the driver to make sure anyone who took over was safe to do so.
Not wonderful, but if you play away from home you must be prepared for local rules.