On a forum dedicated to the London Underground, members have pointed out that plug sockets on the trains are for cleaning equipment deployed when trains are in depots. They recommend not charging electronic equipment as there is a risk of power surge: “If something was directly plugged into it (for example a standard computer, or a laptop without a battery in) the equipment would probably be damaged at any section gaps where the power supply changes from one substation to another!”
This isn't necessarily a standard plug socket, it's intended to be safe for cleaners to use in stations (likely where the at-seat passenger outlets aren't working due to the train's main electrics being switched off) but there's no such guarantee whilst in motion. Many phone chargers won't handle the erratic power supply well. That is a fire risk.