Hey there - the consensus on the decline of the British upper class is more focused on how the slow economic and social changes at the end of the 19th/start of 20th C eroded the position of the aristocracy. While WW1 is certainly a tragic event which decimated the aristocracy (future PM and patrician Macmillan's year at Balliol only had 2/28 survive) it wasn't the main reason for the slip in aristocratic power. I would also argue that it was not really the slip of aristocratic society which enables a socialist society but rather both are symptoms of the economic, political and social changes in the period with WW2 significantly boosting Labour fortunes. Having said this, thankfully the histriography has moved on from the notion of self starting industrialists usurping static aristocratic landowners and/or the idea of an 'extinction' of a class to reveal a much more subtle and nuanced picture.

A good reference work to start with is Cannadine's Decline and Fall of the British Aristocracy while a little dated in some regards offers a robust overview of the changes which reduced the political power of the aristocracy.

In Our Time did a good episode on exactly what you are discussing which would be a good jumping off point as well

Though a little bit off tangent (depending on your definition of aristocracy) is Adam Nicholson's highly readable and broad-brush stroke account Gentry which is another perfectly servicable reference work which conjures up a illustrative account of the ebb and flow of aristrocracy.

Aside from these, other reference works tend to be rooted either in examining many classes, focused on social/political/economic changes or huge chunks of British History - all not quite focused on aristocracy itself.

On another tangental note pretty much anything by Waugh is a highly readable fictional, yet illustrative, portrayal of the neature of the decline of the upper-class: particularly what occupied their conciousness as they dealt with realtive decline.