What policies would I want to see realised? That depends. Have I been elected on a mandate for radical change? My vision for the UK is pretty out there. It's hard to make major constitutional change without enormous momentum. Assuming that I have the average amount of power and momentum afforded to a new Prime Minister, I'd probably only be able to realise two or three large political projects. I think I know what they'd be:
Replace the House of Lords with a Citizen's House, run on the same principle as a lay jury, or of the assemblies put together for British Columbia's 2004 constitutional convention, and the House of Commons with a Senatorial House elected by the Single Transferable Vote electoral system;
Institute a national basic income, such that no individual had access to less than £25,000 a year (£30,000 in London). No labour should be valued at less than £12 per hour;
Abolish all police forces, prisons, and armed forces. Defence and 'crime' reconciliation responsibilities would be devolved to local communities.
To be honest, that's pushing it. It'd be hard not to achieve even one of those goals without severe compromise. Assuming I had infinite power, I'd push on to accomplish the following:
Abolish the City of London;
Bring all major infrastructure (back) into public ownership, including rail, gas, electricity, water, Royal Mail, transport, and communication (i.e., phone and Internet) services;
Destroy the structure of the United Kingdom, devolving power radically to small communities;
Upscale foreign aid, especially to those nations whose wealth was robbed most notoriously by British plunder. Huge aid would be given, without strings attached, to India, indigenous communities in the USA, Australia and Canada, West African former colonies (i.e., Ghana and Nigeria), Jamaica, Hong Kong, China, Singapore, etc.;
Total and immediate withdrawal from British foreign bases in places like Cyprus, and the Chagos Islands, but not places unjustly colonised but which I feel are now effectively too complicated to fairly withdraw from (i.e., the Falkland Islands and Gibraltar). Since I'm a Prime Minister with ultimate power, I wish to make it clear that while I in principle favour a united Republic of Ireland, I'd really rather just pay lip service to the Good Friday Agreement, and keep my nose out of Northern Irish affairs;
Begin a colossal social housing programme, institute draconian rent controls, and issue compulsory purchase orders on the vast majority of second homes. It would be illegal to own a home in London and not live in it. Houses are not commodities;
Give all parties with a certain percentage of the vote an identical allowance, to prevent rich donors from swaying elections;
Firm regulation for those industries with excessive unaccountable power in our society, including the media, banks, and power companies;
Scale up corporation tax, inheritance tax, and income tax on high earners;
Decriminalise and regulate the sex industry, to keep sex workers safe, destroy pimping, and replace the repulsive porn industry;
Decriminalise illegal drugs, to keep them as safe as possible;
Change the way the NHS helps trans people, with much more funding, and more compassionate doctors. All GPs would receive specific training, and we'd push a huge agenda so people could better understand what it's like to be trans, and what it means to transition;
Abolish the borders.
There's so much I've forgotten, but I'm beginning to flag. I hope someone found some of these ideas interesting.
I'd rather not affiliate with a party, but if I must use one as my support base, it would probably be the Labour Party, or the Greens. They are both connected to unions, and the latter has a slightly more democratic structure than the other parties.