Looking at TFL's fare charts, there are two different fares for your journey:
£6 combined national rail & Underground (HOP to LB, change to tube for LB - zone 1)
£4 HOP to Zone1 using Overground & tube (change at Canada Water).

Can't see why you shouldn't be able to buy the cheaper one for that journey.

Of course, all fares are cheaper if an oyster card is used.