Interesting Activity: Ask a Taxi Driver to take you to his/her favorite restaurant

It's social media, designed to be entertaining and make money for the You Tuber. Their goal to continuously come up with things to post. So, for your average, sensible safety conscious tourist, doesn't make as much sense to do it. Though it never hurts to ask a taxi driver about their favorite restaurant or site to not be missed when already in a taxi.

Certainly in London, the drivers you find in black cabs are mainly from outside of London. Most travel in from their McMansions in Essex and Kent to work in central London. Sure, they'll know the places they drop their fares off at but I don't imagine there's many black cab drivers eating out in central London on a regular basis. Again, another sweeping generalisation, but Uber drivers are usually from a different demographic, and while more likely to live in one of the London boroughs, I doubt there's many with the time or finances for such frivolities as central London dining.

As far as being a risk in terms of a fare or safety, fare yes, safety no. Getting into a cab and asking such a question is setting yourself up for being taken for a mug. The little wheels in the driver's head would be whirring immediately with thoughts of how much money they can extract from you by running up the meter. When you book an Uber the destination is already set, so that's not going to work.

As Helen wisely states in the first reply of the post you link to -

Time Out London is a great place to look for restaurants.

That's your best bet.

https://www.timeout.com/london/food-drink