Productivity has fallen for a number of reasons:
1. Output gap, this always happens during a recession where companies tend to keep employing skilled, trained, experienced, but under-worked staff in the expectation that they can quickly expand as the economy recovers.
2. Most service jobs are one-to-one customer facing, so productivity and productivity growth is limited. If each customer takes on average 10 minutes to serve that is only 6 per hour. 48 for an 8 hour day. Competition and margins have been squeezed, we have a glut of cheap unskilled labour from mainland Europe and wages for the semi-skilled and unskilled have trended down due to automation and computerization since the 1980s.
3. Our oil industry is in decline which were very high productivity high value-added jobs. Many of our more productive (than the service sector) jobs have been exported abroad due to our high-cost base compared to 2nd and 3rd world countries, but it has dramatically reduced these countries poverty. Virtually all of our high energy industries have been lost due to the UK plant food policies including chemical, petrochemical, glass, cement and most of the steel industry. The loss of the high energy using manufacturers has generally raised the global amount of plant food produced in much more inefficient 3rd world companies where they don't pay renewables or carbon trading energy premiums.
4. Brexit and the drop, from June 2016 onwards, in the overvalued Pound against the US Dollar and Euro has been a real boost for manufacturing industry growth with sales and exports reaching new highs. This is also reflected in a modest gain in the latest productivity figures.
I personally expect once we leave the EU, long-term growth and productivity will rise where we can concentrate on the 7bn people in faster and very much faster-growing economies outside of the EU, especially once free-trade agreements have been concluded. The virtually no-growth EU will continue to lose global market share at double the speed of the US, which is a good 1st world benchmark to compare it against. EU Commission policy has always been politics, the project and every closer union at the expense of economics. The big problem with this is that economics always long-term wins over politics and normally after the politics have made an economic area much poorer.