There is only one 172 service a week between London & Birmingham at the moment, it is the first one on a Saturday and is coupled to a 168. This is because of concerns raised by guards over the units so it will likely remain this way until a modification is made.
As for the OP, the best train to catch would almost certainly be the 15:55 BMO-MYB, which arrives in London at 17:45. That is booked as a slam door loco hauled service at the moment. There is a brand new power door version in service too but that should be in London at that time of day as it is booked to work the 16:46 MYB-BMO. I have a picture of the locomotive as my avatar, from the other end of the train it looks like this:
(picture courtesy of Google Images)
It's looks close enough to a HST for most kids I think.
The majority of services are currently worked by class 168s. 165s do come north of Banbury, but are generally only used on the BMO-LMS stopping service, the MYB-SAV service and the LMS-SAV shuttles unless there has been a unit swap of some kind. By this time next year most BMO-MYB services could well be loco hauled rakes. I still think the BSW-MYB semi fasts will be 168s though, but I might be wrong.
*edit*
I'd check again nearer the time though, you might find that the diagrams for rolling stock have been changed somewhat due to the Olympics.
