To add to this, there's technically 2 Zero MPs - The main one for EWR is at the former junction south of Bletchley where the original connection to the fast lines prior to the flyover was, and this mileage increases from East to West, despite the line direction having being down from Oxford.
The other is the one
@12LDA28C mentions, at the former Flyover Junction (OXD mileage 0m66ch) that increases from West to East (i.e. In the Down direction) that forms the datum for the connections to both Fenny Stratford and Denbigh Hall South Jn.
Sectional appendix available here:
https://sacuksprodnrdigital0001.blob.core.windows.net/sectional-appendix/Sectional Appendix full PDFs September 24/London North Western (South) Sectional Appendix September 2024.pdf
Extract showing mileages:
View attachment 168598
It's a shame the opportunity wasn't taken to remile the whole line - either continuing the GW mileage from Paddington via Oxford, or a new 0 at Oxford North Jn, but I guess as the old mileages existed and Oxford North Jn - Bicester Gavray Jn was in use trying to change mileages was seen as too big a job and putting in an additional change at Gavray Jn didn't make much sense.