Changing light fitting with red/black uk wiring
It worked when one of each the black and red were in both the neutral and live. But when I turned the light off, the electric trips, and won't come back on. What am I doing wrong?

It sounds like you have wired the switch across the light. When you &quot;turned off&quot; the switch you were actually turning it on and creating a short circuit.
The two red wires need to connect to each other, but not to the light fitting. Traditional UK ceiling roses had a dedicated terminal for this, but sadly most fancy fittings don't, so you have to add your own connector. The tranditional option in the UK would have been a peice of terminal block, but push-in or lever connectors from the likes of wago are increasingly becoming a popular alternative. Personally i'm a fan of the wago lever terminals.
The black wire from the switch should be sleeved red or brown and connected to the live terminal on the light, while the black wire from the supply should be connected to the neutral terminal on the light.
There are various ways to determine which is the supply cable and which is the switch cable, but I suspect the easiest for you is to connect the cables to the light one at a time while capping off the other cable temporerally with terminal block or wagos. Then turn the power on and if the light comes on you have the supply cable.