The Paris metro is cheaper for a number of reasons. Primarily because the relationship between the customer and revenue is less clear. 80% of its operating costs are subsidised by central or regional government, a vastly greater figure than anybody would accept here. Also because they don't have as close a relationship between demand and supply as we do: LU has always put fares up over inflation to regulate use as and when the network was getting too busy at its pinch points and (until the last increase) noticed a fairly elastic response. Paris doesn't do this.

The ?4 for a single is massive, yes, and is part of the Oyster marketing drive, as well as crowd control (if you're fit and able, I don't see how getting the tube from Bank to St Pauls would be quicker than walking anyway). Essentially, RATP want to get people on to their network, and TfL wants to get people off the busy bits of theirs.

And they can't be expected to build a fare structure around people forgetting their Oyster card. A system of reclaims, yes.