I’ve had success with just dish soap - it makes blockages “slide” more easily.

In the last flatshare I lived, I had a particularly annoying combination of a slow toilet and a flatmate incapable of solving any blockages. Whenever I’d see that, I’d go “fuck this”, squirt a silly amount of Fairy in the bowl (I’m talking like 100 ml at least) and usually the blockage would resolve itself overnight.