There's not a standard for this.
What I would do is:
- Install a receptacle hidden inside the cabinet (depending on cabinet style, this could also be installed in the wall if there's no back on the cabinet, or you cut a hole)
- Install a switch on the wall, that controls the receptacle
- Install a second junction box in the cabinet for the low-voltage connection, and run a lamp wire (or similar low-voltage wire) from that down underneath the cabinet to the toe kick (leave it coming out of the top of the toekick to be connected to the LED strip.
- Plug in a transformer to the receptacle, and make the connection to the wire inside the second low-voltage junction box.
You could skip the junction box for low-voltage, but then you'd have to deal with the transformer being connected during construction, and think about how the transformer could be replaced if it ever broke (without having to tear out the cabinets).
The receptacle would also have to be GFCI-protected: so either it is a GFCI receptacle, or it is powered from the load side of another GFCI already in the bathroom. If you do this using an upstream GFCI, you could also run a 14/3 and wire the receptacle as split (so you have both constant and switched power).
I installed a fan in our bathroom to improve air circulation in the house. It is intended to be run all the time at a low setting. We have a motion sensor module in ours that will increase the fan speed (and thus the CFM) for a period of time (settable). I kept the switch on ours, but you're not generally supposed to. Instead, it is common to swap the switch with an outlet, like it appears was done in your case.
A house is intended to have a continuous exchange of air with proper ventilation. If you do not have that, your indoor air will quality will suffer... more CO2 and humidity, leading to mold growth. If mold is detected in a house, a common recommendation is to increase air exchange by installing a continuously-running fan.
If your fan is running all the time at a high rate, it may be possible to install a motion-sensing module. Google the fan's model number to get a manual to find the right part numbers to buy.
If you think your air quality is fine, and you want to make the fan switchable again, you should be able to easily swap the outlet back out for a switch. Generally speaking, you'll want to remove the outlet (with it's pig-tailed wire(s)) and then find the wires that go to the fan... there will be two coming into the box in the same spot which, when disconnected will leave a working light switch, but the fan will be off. One of these two wires will go to one side of the new switch and the other wire will go to one of the existing capped bundles... then you'll need to add the pigtail from the other side of the switch to the other color capped bundle.
Take this all with a grain of salt... I'm not an electrician. Be sure to turn off your breaker when you are doing the changes. If you're not sure which wires are which, it's best to make one change at a time so you can reverse it as needed. I always take a picture before I make any changes. Writing it out on paper sometimes helps me get my thinking straight.
Finally, if you just want to split the difference and be done with it, you could consider just turning the CFM dial down until it is quieter but still moving decent air.
Best Answer
I think it is in theory possible for a device like this to be designed and certified as a shaver socket in the UK. I don't think having output voltage of 5V (or 9, etc) would disqualify it. I do not however think this particular device has been designed or certified to "shaver socket" standards for use in a bathroom and I don't think such a thing exists. You should probably install an actual shaver socket, and plug in a regular USB phone charger cube.
I'm generally not a fan of USB chargers wired into the wall because they quickly become incompatible with the charging needs of newer devices, eg QC2, PD, ETC. A shaver socket and your device charger will give you safety, charging speed and functionality, and future compatibility.
EDIT: Most people can stop reading now. Adding to my answer that "charging speed" and future compatibility are not unbounded. A shaver socket may, by the standard, deliver at most 50W but a manufacturer may limit it to as little as 20W within the standard for shaver sockets. A 20W shaver socket should easily feed a USB charger that comes with a shaver or toothbrush. It will not however charge most new (in 2021) smartphones at their maximum speed. A 50W shaver socket will charge any phone or tablet that I am aware of (in 2021) at its maximum speed, along with any other QC2 or QC3 device. But QC4 and future standards require more than 50W and PD is already capable of 100W output (which will draw more from the AC source) so don't expect to charge your 15" gaming laptop in the bathroom (as if you needed another reason not to leave your laptop in the bathroom).