Indeed the pricing policy of Chiltern does seem skewed to attract London to Birmingham traffic, with bargain advance tickets available, whereas shorter distance travellers get so such benefits!
There are some very good prices available for advance fares between Oxford and London (for example), so I'm not sure that's entirely true.
again not their core market, try getting a good price from Princess Risborough or other places in that area.
There was no mention of their “core market” (definition?) until yours. Oxford <-> London is a shorter distance than Birmingham <-> London, and does have the benefit of bargain advance tickets being available, which is all that I was noting. A quick check suggests that there are indeed no advance singles available between Princes Risborough and London, sure. I guess Chiltern figures it doesn't need to discount that particular journey, perhaps because there isn't as much competition there (Oxford <-> London has both the GWR route, and the express coaches, each of which can offer a very good (and often superior) alternative, depending on where in Oxford and London your endpoints are … and so price competition is a straightforward way to keep themselves in the game (GWR has certainly had advance fares available between Oxford and London for years, since well before the Chiltern service started)), but, whatever the reason, it's their commercial prerogative to offer, or not, advance fares between station pairs they serve, surely?
If we are now diverting to trying to define “core market” though, then I'd suggest that the quantity of trains serving Oxford and Birmingham, as well as their length (and their mostly using “Mainline” or loco-hauled stock) indicates that services from Oxford or Birmingham to London /are/ at least two of their core markets.
There are 2 Chiltern trains per hour between Oxford and London in each direction through the day off peak … which is the same number as Princes Risborough, as far as I can see.
I won't go on, but using the term “core market”, without defining what it means, is a bit … meaningless.