How can a circuit be designed with a switched live connected to multiple PIR motion sensors? [UK]
I answered your core question here. Just go into photoshop and copy-paste as many more sensors and lights as you please.  Go read it, but let me re-color that for UK.

The only trick is going to be UK wire and cable.  You need 3 wires:

Always-hot to supply sensors
Switched-hot for sensors to send power to lamps
Neutral for everybody.

So you need 3-core + earth cable.  But while North American /3+gnd cable accommodates neutral, I note that common UK 3-core+earth cable does not.  You need a light-blue neutral.  Your 3-core appears to be black brown gray.  I don't know if you're allowed to get light-blue tape and re-mark the gray wire to be neutral, or whether you need to source special 3-core that has a blue wire.