Greetings! I highly recommend that you follow-up the recommendations within this comment by scrolling through the far more comprehensive and inclusive AH booklist on Britain. Here however, to get you started, are a few areas to start that have helped me in my research on 20th century Britain:
Reading:
- The Empire Project: The Rise and Fall of the British World System 1830-1970 by John Darwin (2009, ISBN 9780521302081). This book is an excellent deep-dive of about 600 or so pages (with great references and further reading at the end) about how Britain and her Empire weathered the storms of the tumultuous late 19th century and early 20th century, before finally declining after the Second World War. Darwin's work is far more geopolitically minded however, so do not expect a fleshed out investigation of actual events/persons who were crucial to 20th century Britain's development. He does however, help you out by explaining the basic context behind key events, so you are not just lost in the historiographical arguments without knowing any prior knowledge.
- Admirals: The Naval Commanders Who made Britain Great by Andrew Lambert (2009, ISBN 9780571231577). This book is much more of a "great men" one when it comes to British figures, and more specifically, the British admirals who served in the Royal Navy from the time of King George V to Winston Churchill. The last three admirals on this list do technically fit into the 20th century category: David Beatty, John Fisher, and Andrew Cunningham. Lambert is a pre-eminent naval historian, and his source-work is most thorough. Very contextual in nature and each chapter basically "assesses" the key strengths of each admiral by serving as a mini-biography of their life and service in the "senior service" which represented the strong arm of the British Empire. Definitely a lighter read than Darwin's one above, but still worth a look nonetheless.
- The British Empire: A Very Short Introduction by Ashley Jackson (2013, ISBN 9780199605415). This entry in the Oxford Very Short Introduction series is great for people looking to go beyond the past century of British history, and touches on a nice spread of thematic topics about the development of the Empire from its origins in the 1600s to pre-eminence in the 1800s and of course, decline in the 1900s. Obviously due to the nature of the work, Jackson does not go too far in-depth with any particular event or figure, but does provide a nice overview of the "scenery" if you will of the British Empire's rise and fall throughout three centuries.
Videos
I hope OP does not mind sitting back and watching hour-long lectures, because that is the majority of recommendations I have for British history in terms of publicly accessible videos. More specifically, Gresham College has a wonderful series of lectures by renowned British historian Vernon Bogdanor, covering everything from British events of the 20th century to leaders and figures of the era. Bogdanor spends a lot of time on the contextual details before getting into the more historiographical arguments, so the lectures are definitely an interesting entry point to most (if not all) topics of key importance to Britain in the past hundred years. Whilst the full playlist can be found here (it does contain some more modern lectures strictly on British politics post-1990s, and some purely political theory), I have linked below some particular favourites of mine:
Hope these recommendations help, and feel free to ask me any followups as well!