There's a McKinsey study I no longer have access to summarised here:
https://www.mckinsey.com/global-themes/europe/why-is-labor-productivity-in-the-united-kingdom-so-low
There was a piece in the Grauniad not too long ago about how a service-based industry is appraised incorrectly by such a basic measure with no nuance, amongst other things.
There's very low unemployment in the UK and that's been the case for some time. When unemployment is this low, it becomes very difficult to employ good people and inefficient staff hold their jobs longer. This one is personal experience.
Minimum wage makes it very difficult to differentiate wages and attract better staff at the manual end of the scale. Meaning more people are required to perform the same tasks and/or management time is taken up instructing them how to complete simple tasks. Also personal experience.
If job security and all that fluffy lefty nonsense is the cause of productivity, why is the US so far ahead of the rest of the world? The closest thing we have to a complete lack of workers' rights in the Western world is storming ahead without comparison. I don't know a lot about the economy in Japan, but I know productivity is terrible and I'm fairly sure job stability is really high.
A minor factor I've seen floating around is the infrastructure issues we have here and I thoroughly agree with that. Try driving to Birmingham from anywhere in the South East without driving on the M25. You end up on what is apparently a major arterial route, which is only 2 lanes wide! Most of our major A roads are at least 2 lanes too small in each direction, the West section of the M25 needs at least 2 more lanes each way, the North and South Circulars both need a couple of lanes too. There are even sections of the M3 (a fairly major motorway) which are 2 lanes wide each way. I'm reasonably lucky, I can schedule all my lengthy phone calls for when I know I'll be in the car, but for some people that's just wasted hours every day.
Obviously education plays a big part and I've been fairly vocal in the past about the sorry state of our education system. Too many poor teachers bouncing around the system (this opinion is from teachers). Too much "deferred success". Forcing non-academic types to study subjects they will never comprehend instead of giving them job experience. Forcing academic kids to share classes with those who cannot and will not attain the required grades. Many classes are little more than an exercise in zoo keeping, leaving academic kids with no chance to show their ability and stretch themselves. Kids who were never cut out for school, let alone college or university ending up dragging themselves through a Sports Science degree because Tony Blair thought 50% of kids attending university sounded like a nice number without ever stopping to think of the consequences. I'm sure I could think of plenty more but it only makes me angry so I try not to.