There is a fundamental difference between UK and USA systems of power supply for domestic and similar purposes.
In the USA power is typically supplied via TWO live or hot conductors and a common or shared neutral. The voltage between either of the hot/live conductors is 120 volts nominal. Circuits for lighting and small appliances are connected between one hot conductor and the neutral and receive 120 volts.
The voltage between the two hot conductors is either 208 volts or 240 volts. Large appliances that are designed for 208/240 volts are connected between the hot or live conductors, and depending on the design of the appliance may or may not also require a neutral connection.
Most power failures are total and no electricity is then available. However it can happen that ONLY ONE of the two hot conductors is live, in that case lights and appliances connected to the still live hot conductor will function, but other lights and appliances connected to the dead conductor wont work.
Under such "partial failure" conditions, there is a significant risk that the voltage on 120 volt circuits may be MUCH higher or lower than intended. This can be DANGEROUS and may cause damage, I advise against use of electricity under such conditions. If you must, be very careful.
In the UK, power is distributed along a street as 3 phase, 4 wire with 240 volts between any phase and neutral, and 415 volts between any two phases.
Each house is normally connected to only ONE phase, selected at random. The supply is TWO wires only, not three as in the USA.
This single live connection is either working or not working and there is no question of some circuits working and other circuits not working, as might happen in the USA.
If only one or two phases of a UK 3 phase supply have failed, then neighbours may have power when you do not.
A very large house might have a three phase supply, in which case a partial failure IS possible. 3 phase to a dwelling is rare in the UK.
In relatively rare cases in the UK, the voltage might drop to say 110 volts. A PC with a multi voltage power supply might work normally, as will SOME types of LED light bulb. Most other appliances wont work, and this can give rise to reports that "only two lights worked"
Some appliances are liable to damage by this reduced voltage and should be turned off. If in doubt turn off at the mains.
The above is a simplified version of UK/USA arrangements, for example correct earthing/grounding is vital for safety, but not directly relevant to this discussion so has not been referred to.
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