Rejoin the customs union and single market
Tqx reform (huge simplification to remove loopholes, rationalising rates to remove bottlenecks at £50k and £100k earnings thresholds which is damaging productivity, automatic uprating of thresholds with inflation unless an act of Parliament changes this, raise capital gains tax to income tax levels but scrap inheritance tax, lower personal allowance but lower basic rate income tax also, remove penalisation of single people/ parents with household-based allowances/ thresholds, NI to be charged to pensioners, crackdown on self employed black market cash income, CMS to be processed as a tax and at far increased rates, big incentives for R&D in key UK stategic growth areas, no tax to pay for employer or employee during apprenticeships, lower corporation tax for small businesses but levy a minimum % on all UK revenue of large corporations if UK profits assigned are lower and crack down on transfer pricing).
Double or triple the state education budget and total reform of the national curriculum plus opportunities for retraining later in life. Proper provision in schools specialised in different subjects as elsewhere in Europe so children can have a rounded education but focus on their specific talents/ interests and have schools that meet their individual needs properly. Big focus on technical colleges as in Germany with links directly into employment. Total reform of the NHS to a European style system that can provide adequate care for a not hugely dissimilar percentage of GDP. All civil servants to be given a large pay catch up to make up the pay cuts of the last decade so that public services can be properly staffed (single market and CU will also help with this. Brexit tax loss to the treasury is already more per year than the entire state education budget).
A proper industrial strategy with long-term plans for food, energy and water security. We should be investing in hydro power, solar panels on every house, proper infrastructure in terms of internet and power network capacity and storage, battery technology for EVs and power storage and supply chains with the minerals required for that, advanced farming methods that can generate far more food sustainably from the same land capacity, building new reservoirs (water will become an extremely precious resource very soon), focusing on UK growth sectors like tech, pharmaceuticals, research, and also far more investment in AI. Investment in all of these can be achieved through Government/ private partnerships as long as point 1 is addressed (single market and CU). Far more support for small business start ups. Some proper thinking also about how the economy will be managed with the advance of AI which will cause turmoil on a scale thaf few people seem to be acknowledging, or how fast this will happen: millions of people in the UK will become unemployed. What is the plan for this? Our whole social structure will have to change. Plus there will be hundreds of millions of climate refugees over the next few decades. There appears to be no plan for any of this, at all.
Complete reform of the state pensions system so that the amount of contribution is reflected in pension income, a long-term plan to phase out the current ponzi scheme, also root and branch reform of other benefits so that far more are contributary (and therefore more generous like elsewhere in Europe but limited in time), however disability benefits to be at least doubled to give disabled people some dignity.
I think actually I'd add a sixth - introduce PR, reduce the number of MPs to 1/3 but triple their salaries so you can actually attract competent people who are not either a) wealthy already and using it as a stepping stone, or b) simply not capable or experienced enough to do the job. I'd also make a rule that no politician can be in Parliament for more than three terms, to avoid "career politicians".
The problem is that all 5 of these - while essential and extremely obvious - require long-term plans, actual detailed plans for implementation not just announcing "targets". And long-term vision, practical ability to get things done, the willingness to do what needs to be done not just make "popular" decisions and think about the next election. So I doubt any of it will happen, and the UK will continue to decline and have falling living standards. Other countries are doing such things and if we won't, we will get poorer and poorer, as certainly as night follows day. Ultimately living standards in the UK will not rise unless productivity increases. The above steps create the foundations for that to happen, cut wastage, put money in the right areas of investment and therefore will more than fund themselves over time. If you don't do any of that, then the economy stagnates, GBP will gradually lose more value and the large imports of essential goods like food and fuel that the UK will still be dependent on will become increasingly unaffordable. People should be very, very worried that the balance of payments deficit has quadrupled since Brexit. That is not sustainable. It's all basic economics really.