Changing light fixture with red/black UK wiring using only black wires
Standard UK loop in wiring. 

The 3 reds joined together will be live in, live out (to the next light) and live down to the switch.

The 2 blacks joined together willl be neutral in, neutral out.*

The black on its own should be sleeved brown (was sleeved red) for switched live.*

The new light connects to * and * (and earth, if it's not doiuble insulated). 



from

https://www.practicaldiy.com/electrics/lighting-wiring/light-wiring-loop.php