The more expensive ticket for travel via London is routed "✠ANY PERMITTED". This means that it is not restricted solely for travelling via London, and is also valid for avoiding London.
However, the cheapest option for a through ticket (NB a combination of tickets may be cheaper), would be to obtain a "NOT VIA LONDON" ticket and then to obtain a change of route excess for the portion which you want to use via London. This excess will only cost half the difference between the respective routes of ticket, and so it's a cheaper option than paying for the validity via London both ways if you aren't using it both ways.
A change of route excess cannot be obtained online and must be obtained either at a ticket office or onboard the train. There is no penalty for obtaining it onboard the train. However, note that Chiltern services south of Banbury usually carry only a driver, and are not usually scheduled to have a guard. Thus you would be very unlikely to encounter a member of staff who could sell you it onboard (RPIs do not have any ticketing facilities as far as I know).
Furthermore, even if this were not the case, Chiltern guards are allocated ticket machines that, as far as I am aware, are unable to sell tickets or excesses which are to be valid for a cross-London (Underground) transfer, as they only have "loo roll" stock in their machines, which is not compatible with London Underground barriers. Quite why this was done for a TOC where virtually all services are to or from a London Terminal is beyond me... (well, it's probably a very badly thought through cost-saving measure)
As a result of all of the above, it would therefore probably be easiest to obtain the excess before travel, as you may have to pay for the cross-London transfer yourself if you cannot get it before starting your journey; whilst there is a ticket office at Marylebone, they may be unable to, or refuse to, sell you the excess.
