Assuming you can change anything without restrictions, I would change the voting system to make it fair for lesser known parties to grow. Right now it's just two parties who have chances to get into power alone (Labour and the Tories), and other parties like SNP, UKIP, Green and Lib Dems wont manage it. I'd make it so people are able to pick their top two, if there isn't a better term to use. I.e. if (and thats a strong if) a person desires, they could vote for both UKIP and the Green party, with each vote having the same amount of power. The only problem I've found is if people do not want to pick a second party, but still want the same power as everyone else. Voting twice for the same party wouldn't be fair, as it goes against what this system is for, and only having 1 vote means you've used half of your voting power.
Other thought is for a ranking system, i.e. entering your top 3 selections with number 1 having the most power and number 3 having the third most power (any not voted for have no power).