BBC discovers that Oyster users can be overcharged for incomplete journeys
 

That seems to mean that it would charge my Oyster card, my contactless
debit card, and the other Oyster card that I bought earlier in the day
to give to a friend.


It's also complete and utter nonsense, so don't waste your energy getting
annoyed about it!

There is an interesting research document about RF distance ticketing at

http://www.dft.gov.uk/pgr/scienceres...entsystems.pdf

The main conclusions seem to be that the obstacles would be primarily commercial objections and passenger acceptance rather than technical issues.