I've only recently started looking at single journey prices as my current working practices make a season ticket useless to me. I was amazed at the difference between a paper single from St Albans to London and using contactless - particularly at off-peak times! The contactless fare is even cheaper than the discounted railcard fare. Not that I'm complaining but it does seem surprising, any idea why?
Single tickets in the UK have traditionally been very expensive compared to returns and this has been especially true for off-peak fares. Sometimes the off-peak single is only 10p less than the return! Sometimes an off-peak single does not exist at all.
TFLs oyster/contactless system (in this particular case it's not currently possible to use Oyster) on the other hand has no concept of return fares, so if they priced the fares on it the same as the paper single tickets hardly anyone would use it. The system traditionally used zonal fares, but recent additions to the system have been priced at roughly half the corresponding paper return ticket.
In your case the fares are
Paper off peak day return - £16.60
Paper off peak single does not exist.
Contactless off peak - £8.30
Paper anytime return - £22.50
Paper anytime single - £13.20
Contactless peak - £11.20
Further complicating matters contactless has different peak rules from the paper ticket. The paper off peak day return forbids travel on "trains timed to depart after 04:29 which arrive at, or depart from; any London Terminal, Farringdon, or Kensington Olympia before 10:00" but it has no peak restrictions in the Evening. On TFL contactless the peak period for the outward journey is 0630 to 0930 and for the return joruney is from 0630 to 0930 and from 1600 to 1900. I can't find any official documentation, but my understanding is that TFLs peak periods are based on touch-in time and there is a "grace period" such that the actual period where you are charged peak fares is shorter than the advertised period.
Putting it all together.
* If you are not returning the same day then you should use contactless.
* Otherwise if you have a railcard that is compatible with your times of travel you should use a paper day return ticket (peak or off peak as appropriate) with your railcard.
* Otherwise if your train into London departs after 04:29 and will arrive in london before 10:00 you should use contactless.
* Otherwise you should use a paper off peak day return ticket.