Actually London works because the barriers are required to be manned. So if 1% of users have crazy tickets (e.g. some international through exist that are valid across London for say Birmingham to Paris) the human just glances at their ticket and lets them through.

A typical barrier in London accepts: National Rail (orange) tickets that used to be labelled "Rail Settlement Plan", Oyster (a contactless travel pass used for most public transit in London), the various contactless payment standards (for your VISA etcetera) and its own pink tickets. But a human is always there if you have something else.