In reduced visibility, be it fog, rain, snow, smoke, smog, etc it is way more visible than a tail light, so you are seen from the rear at much greater distance than just a tail light.
Hey bro, I'm with you :boink: Went through the same pain with my 2012 SR5 that is going to Poland. I ended up with designing and making my own rear fog light . You may check the thread I started some time ago:
http://www.toyota-4runner.org/5th-gen-t4rs/204372-2012-sr5-rear-bumper-question.html
The other option I considered was cutting the rear bumper cover and install ready made "reverse-backup" lights (someone did this to his white pearl truck), but put red LED bulb instead of white.
Yes, it is Euro requirement. I got some questions from this forum: "WTF is rear fog light

". BTW single legal fog light here looks weird, really like a broken tail light.
I'm not sure if in UK is the same as in Poland:
The rear fog must have the same brightness as a brake light but must be at least 10cm away from tail and brake lights. so using dual intensity tail light bulb does not work.
The other requirement is that it has to be no higher than 100cm above the ground and no lower than 20cm. Because T4R sits high above the ground the 20cm minimum is not a problem, but using tail lamp housing for rear fog will not fit into 100cm limit (tail lamp in T4R is higher than 100cm above the ground).
The last thing is that if single rear fog lamp is used, it must be on the left side (oh, in UK you drive on that side, so maybe it is the right side). It can not be in center. That simple eliminates the option to put the rear fog lamp in the hitch receiver.
So here is my rear fog light (In US I plugged it into trailer plug so they work as stop and blinkers, which is legal here).