The normal way to wire in the UK is to use a ceiling rose as a junction box and as a result you would have one cable with three wires in the cable. The Green/Yellow is always earth and often it is a bare wire with sleeve. Where double insulated back boxes and switches are used often the wire is folded double and the sleeve covers whole earth wire ready to use if switch is changed to metal type.
In which case you would have two cables and by right either both same colour or one has a sleeve to make it same colour however common to find these missing. If wired that way then one wire to Com other to L1 or if no Com the L1 and L2.
However with wall lights and extra low voltage lights the ceiling rose can't be used as a JB so two cables are taken to switch and either a double pole switch is used or the two blue wires (could be black in old house) are connected with a connector and the two brown (red) go to switch as before Com and L1.
Because there are different methods of wiring where there are no plans only method is to ring out and find where each wire goes.
Where two switches are used for one light you will also get two cables one going to light and other to second switch. Often the inter switch cable will have three cores but not always. Years ago it was common to borrow line connections from another lamp with second switch so they would need ringing out.
Someone has fed the light with a feed directly to the switch with the 1st cable, then onto the light fitting with the second cable. In which case you connect the Red (or Brown) of each cable to the switch (one in each terminal, top and bottom) and connect the blacks (or blues) into a connector 'choccy block' and leave it floating in the switch box.
This effectively takes a live and neutral to the light (which is all it needs to work) but gives tha facility to 'make' and 'break' the live cable providing a means of turning it on and off.
Make sure the earths are connected together in the switch box of course.
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