Britain has a first past the post electoral system in which people vote in constituencies of about 85,000 people to elect MPs and the government. This means that the total number of votes won by a party is less important that your spefic concentration of support. Winning a constituency by 35,000 votes has the same effect as winning a constituency by one.
In a two party system, this would matter less but Britain of course is massively divided and has a multi party system. Only labour and Conservative will have the power to form a government but several other parties are competing for seats and for votes.
Sooooo... tactical voting really matters. Some constituencies are safe for a party but what do you do in a marginal constituency where your favourite party can't win? Do you vote for a lesser evil or do you 'waste your vote' l. If I lived in a Conservative Lib Dem marginal then it would be a huge dilemma. I hate the Lib Dems but I hate the Conservatives more and I don't want a hard Brexit.
So many dynamics are at play because people will be considering lots of different policies as well as the all important Brexit question.
I think people should be free to vote for their first choice party and that we need a proportional representation system.