Holywell is the name of one of the parishes of Oxford.
Holywell Street is a road in central Oxford next to Hertford College which Evelyn Waugh attended in the 1920s. It is generally assumed that Hertford is also Charles Ryder's college.
The Parks are the University Parks which are situated mainly on the west bank of the River Cherwell.
Here, Mesopotamia is not the region between the Tigris and the Euphrates, but a narrow island in the University Parks. Prior to 1926, a ferry operated from the island, but it has now been replaced by a footbridge.
North Oxford is a desirable residential area and the site of several private schools.
Map showing Holywell Street (bottom) and the University Parks (top):