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I am in the UK (240v system).

I have a two gang light switch in my new house. Each switch controls a separate light, and it is the only switch which controls each light (one way switches). I want to replace the switch, so I unscrewed it from the wall and I don't understand how it is currently wired.

I expected each switch to be wired individually: the hot (brown) wire to the live (common) terminal, the neutral (blue) wire to the L1 terminal, and no ground (the switch is plastic). This is how the equivalent single switch is wired in the house.

What I found is that the the back box contains two neutral wires connected together with a terminal block, but not attached to the switch, and four brown wires. The first brown wire enters the live terminal on switch 1 and the second brown wire enters the live terminal on switch 2. The third brown wire is connected to L1 on switch 1, and the fourth brown wire is connected to L1 on switch 2. There is a bridge wire connecting the third and fourth brown wires, so they are always on the same circuit and the L1s are connected on both switches.

How does this work, and why are the neutral (blue) wires not attached to the switch at all?

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    Do your old switches need neutral? Do your new switches need neutral? How many circuit breakers do you need to turn off to de-energize the box? And lastly how many cables enter the box?
    – Harper - Reinstate Monica
    Commented Sep 8, 2021 at 0:52
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    Can you post photos of the inside of the box you have questions about?
    – ThreePhaseEel
    Commented Sep 8, 2021 at 1:00

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Blue insulation does not mean that it's a neutral wire. It might be neutral, but it's not necessarily, and is almost certainly not neutral if it's connected to a switch - light switches don't (normally) switch neutral.

So what you have with your single switch is a "switch loop". The power comes to that light up in the ceiling, then a pair of wires runs down from there to the switch where the brown is (probably) the 'always hot' and the blue is (probably) the 'switched hot' returning back up to the light. In this case the neutral stays up in the ceiling and doesn't make an appearance at the switch.

With your 2-gang switch you have a different arrangement.
In this case it appears that you have power arriving at the switch box on a brown/blue pair (always-hot & neutral) and also leaving the box on another brown/blue pair - these are your 3rd & 4th brown wires which are bridged across the switches.
This outgoing pair goes on somewhere else in the house to power something else and is not affected by these switches.
The switches themselves each have their own brown switched-hot running back up to their respective lights.

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