This would require a POD of 1918-1923, because ww1 lessons would be crucial. There were lots of things that should have been done during the interbellum, including:
- Land reform and LVT, combining with state-backed credit facilities for farmers and state-assisted cooperative marketing to develop agriculture. LVT would also encourage productive investments by help reducing ground rents.
- Solving industrial unrest between workers and owners (either based on Mond-Turner talk or Britain's Industrial Future).
- Limiting Geddes Axe, at least Education Budget must be safeguarded. Education is the key for national development. After that, raise school leaving age, expand university and technical education, and forge a link between schools and industries.
- Public work programs to improve national infrastructures, especially electricity supply, railway electrification, telephone system, roads, housing and slump clearance. After ww1, Britain seriously lagged in these aspects. These would also help solve the mass unemployment problem in the North and boost national economic growth either directly or indirectly.
- No return to prewar Gold Standard.
- Rationalizing and modernizing of staple industries in the North, especially textile, coal and shipbuilding via tax and subsidy incentives.
- A National Investment Board to control and facilitate domestic investments by channeling funds to industries and National Development schemes. Capital should be directed to domestic investments rather than be allowed to flow to places like USA or South America.
- No appeasement of all kinds. This would butterfly away OTL ww2 (it would be a quick war instead).
For post 1940:
- Develop a strong link between military and civilian R&D like in the US. Allowing military tech to freely spill over civil industrial sectors.
- More efficient military spending.
- Nurture the computer, jet, life science and nuclear industries via subsidies, government orders and export discipline (with a bigger emphasis on exports to Europe) (the third one is crucial to prevent businesses from abusing subsidies and hiding behind protection, as well as to force them to continuously improve themselves since international market would be less forgiving). Encourage ICI style M&A to allow these industries to take advantage of economies of scale (not some kind of forced merges like IOTL).
- Computers should be quickly introduced in various civilian government tasks, first for those that relate to statistics and numbers, as soon as the war ends.
- Sending ODA to less developed countries and dominions like India, Eastern European countries, South East Asia... ODA for infrastructure development would include clauses that require using British equipment and components. This strategy would help strangle Britain's potential competitors in these areas from the beginning, while also generate export orders for British firms.
- Demanding foreign MNCs to open R&D facilities or to sell their technology at a low price if they want to invest in the UK, so that Britain would benefit from foreign technology (beg, borrow and steal).