UK shaver sockets are not a lower voltage, but the ones in bathrooms include an isolating transformer and thermal cutout so that neither wire of the shaver has any voltage to earth (so a shock to earth is impossible) and the socket cannot be used for more powerful appliances.
As bathroom shaver sockets include a transformer, it is easy to incorporate an additional tapping to provide a 115 volt outlet for shavers for visitors from 110-120 volt countries.
The shutter interlocks so only one socket can be used at a time
- to prevent the transformer being overloaded
but more importantly
- if two shavers were connected and became faulty it might be possible for a shock to occur from the shaver in one hand to the shaver in the other hand. This could be full 240 volts and a hand-to-hand shock would cross the heart line, and such shocks are much more likely to be fatal.
Some styles of sockets have a single socket outlet and a changeover switch to change the voltage, eg
(note that if both switches are pressed down accidentally, the lower voltage is selected)
Transformer-isolated shaver sockets are the only type of socket allowed in bathrooms in the UK (or at least, within 3 metres of a bath or shower, and most British bathrooms aren't big enough to have 'ordinary' sockets far enough away) but shaver sockets without transformers can be used in bedrooms and dressing rooms (the 3m distance does not apply to handbasins):