Everything I read seems to focus on the Blitz taking place in London from Sept 1940 to May 1941, at which point the Germans redirected their forces. But the war didn't end for another 4 years, and air raid protocols seemed to be in place for the rest of the war in the UK, and there's at least one incident (Bethnal Green tube collapse) in 1943 that occured because of too many people in a tube station during an air raid alert. But Wikipedia doesn't even list a single air raid anywhere in the UK much less in London after 1941. Were they so small as to not be notable? Actually non-existent? The allies were bombing their fair share of Axis targets so it's surprising to me that there's not a single air raid listed.
Can anyone suggest somewhere to learn more about those incidents, if they do exist? Everything I can find just seems to lead back to the Blitz.
There were air attacks over the rest of the war, though much smaller in scale than The Blitz. This was largely due to the majority of German bombers being redeployed to the Eastern Front, though British air defences also improved considerably.
There were two particular campaigns, both prompted by the increasingly heavy Allied bombing; in April 1942 Exeter was attacked marking the start of what became known as the 'Baedeker Blitz', targeting historic (and weakly defended) cities. Bath, Norwich, York and Canterbury were subsequently attacked; see a previous post for a little more on the target selection. January 1944 saw the start of Operation Steinbock, also known as the 'Baby Blitz' or 'Little Blitz', primarily targeting London. Neither campaign inflicted significant military damage, though there were around 3,000 civilian casualties, and both were costly for the German Air Force. 1942-43 also saw many 'tip and run' raids - small numbers of German fighter-bombers attacking coastal targets at low level, again with little effect.
The last heavy attacks were unmanned, employing the V-1 flying bomb from June 1944 and V-2 rocket from September 1944. Considerable effort was put into defence against the V-1 or 'Diver', including radar proximity fuses for anti-aircraft shells and Gloster Meteor jet fighters; only around a quarter of the ~10,000 V-1s launched reached a target (primarily London), though that still resulted in about 6,000 deaths. There was no real way to intercept a V-2, they killed another 2,700 civilians before their launch sites were overrun in March 1945.
For an overview of the entire bombing campaign Richard Overy's The Bombing War: Europe 1939-1945 is excellent, though there are only a few pages specifically on the post-1941 German campaigns. Alfred Price's Britain's Air Defences 1939-45 from Osprey gives a brief overview; Basil Collier's official history The Defence of the United Kingdom is online at HyperWar; Colin Dobinson's AA Command: Britain's Anti-Aircraft Defences of the Second World War goes into rather more detail on that aspect. The Air Historical Branch narratives are available from the RAF, volumes V and VI of the Air Defence of Great Britain cover the post-Blitz period. Chris Goss' Luftwaffe Fighter-bombers Over Britain: The Tip and Run Campaign, 1942-1943, Jan Gore's The Terror Raids of 1942: The Baedeker Blitz, and Ron Mackay's The Last Blitz: Operation Steinbock look specifically at those campaigns. I'm afraid I haven't got much from the German side of things, other than Horst Boog's chapters in volumes VI and VII of Germany and the Second World War.