Codified constitution, gun legalisation, legalise file sharing, take back power from Europe and more devolution incl. an English Parliament. Drug policy reform marijuana declassified
Less of course, I see no reason why law abiding citizens should be prevented from access to such weapons.
Edit: And to add to my list, if the Queen is dead by the time I'm PM. Have a referendum on the future of the Monarchy and an elected House of Lords
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I would add to that proportional representation and I would abolish peerage and the house of lords instead of making it an elected chamber.
I'm not sure about the English Parliament though; it would render Westminster rather unimportant.