I spent most of book 1 of the Portia Adam’s adventures, referring to the College she attended as ‘Queen’s College’, a made-up name that I intended to replace with a real college when I got around to doing the research.
That time is now, and I believe I have picked
Somerville College 1907, Oxford http://www.francisfrith.com/oxford/photos/somerville-college-1907_57394/
Somerville College as Portia’s educational centre. As part of Oxford College, it is a little out-of-the way – an hour by tube from Baker Street to Oxford, but it fits the bill in every other way.
It was a degree-granting institution (since 1920) and didn’t admit men until the late 1900s. The principal at the time Portia would have attended was named Helen Darbishire and in the 1930s, the College was actually expanding (despite the Great Depression).