How to add new light to existing 2-way switch

Question

• I currently have a 2 way light.
• I am working with the old Black and Red wires, not the current Blue and Brown. (I am in the UK.)
• There are 2 switches to this 1 light. Switch A and Switch B
• In the back of both switches, there are 2 wires coming down the walls. 1 wire is a twin and earth with the Black and Red, going to L2 and L1 respectively. And a second wire is a single core going to Common.

Constraints:

• I am unable to run a new cable from the existing light fitting to the new light fitting.
• I am only able to access the cables in the ceiling above Switch A.
• I am unable to run a new cable from any of the switches.

I have fitted the new light fitting and run a twin and earth from this new light fitting to where the Switch A wires are in the ceiling. (L1, L2 & Common)

1. Is it possible to add a second light to these switches?
2. If so, how do I do this with the constraints above?

Answer 1

Because UK lighting carries the power in the ceiling not in the walls [except for newer installations] then you can't jumper out of either switch, or the cabling in between, to do this - you have no neutral so the result would be a series connection through both lamps.

You need to be able to run parallel from the existing ceiling rose.

Google found this diagram on Pinterest, linked from https://flameport.com [though I can't find the relevant page.]

You can see from this that it doesn't matter how the switches are wired, one, two or three way, because the net sum of any/all those switches arrives back in the rose, ready to parallel off to any other rose. Your parallel takes the switched live plus a neutral [& earth] to the next lamp.
This is in new colours, but I'm sure it's simple to translate. There's usually only a 10% chance the installer actually put a brown or red sheath on the switched live, but you can tell by context, because all ceiling roses are like this 3, 3, 2 pattern for neutral, loop & line.

Because of the way UK lighting circuits work, you may already have a parallel matching the cable marked 'IN', unless this is the last fitting on that circuit. Leave that in situ [ignore it completely] as that will go off to another light in another room. They're all wired together, usually one circuit/fuse/breaker per floor, upstairs/downstairs.

Late edit
Of course, when you get to your new rose, you simply use the same terminals as the rose you came from, basically to each side of the new lamp - neutral & line [& earth]. You don't need anything else in there.