When buying a UK train ticket, if one of the allowed routes for it involves changing stations in London, then then the route on the ticket will be printed with a dagger (†; described in publications as a "Maltese cross"). The ticket will then allow you to (at no extra charge) use the London Underground to travel between the stations on your cross-London transfer.
In France, when buying a train ticket which requires a cross-Paris transfer, you are issued with one ticket from the origin to the first Paris station, then a second from the other Paris station to your destination. The ticket is not valid on the RER or metro, and you therefore have to buy an additional ticket on arrival to get you across Paris. (Or walk, but that's only practical in a handful of cases)
What is the reason why the UK system manages to include the cross-London transfer in its tickets, while the French one doesn't for cross-Paris ones?