Make the biggest difference to the UK, or most benefit to the people who live here ?
Probably I would say taxation reform. At the moment property/land is not taxed anywhere near as much as income. This means a lot of rich people hold large amounts of land which is not taxed. We need higher CGT and lower income taxes, and replace some income tax with property taxes. Essentially we need to incentivise work, and disincentivise rentier activity like landlordism.
Next I would say house building. At the moment incentive to work is low, because income taxes are high and property prices have been elevated too high. We need to build more houses both private and social and tax ownership of them (as above) to bring down property prices so people in normal jobs can afford housing. The idea that police officers, teachers, nurses etc cannot earn enough to afford their own home without subsidy is ridiculous.
These two changes would make a huge difference to the UK and to the average persons quality of life. If people didn't have to spend loads of money on mortgages, they would have more money freed up for other spending such as holidays. High house prices are a cancer on society - they are making it impossible for people to have a decent life because they either do not feel secure enough to start a family due to lack of housing, or they just don't bother working because there is no chance of them ever being able to afford their own home.
Now, the chances of the above two things happening, close to zero. Because of the old money vested interest as well as the idiots who spend their time promoting ever increasing house prices. Plus changes like this would probably most likely impact the old, and the young seem largely indifferent to voting, so it would be unlikely these policies would get voted in.