Who takes over the country when parliament is dissolved in uk?

Taken from the UK Parliament website (see related links):

The Government does not resign when Parliament is dissolved; essential business must carry on and government ministers remain in charge of their departments until after the result of the election is known. Only then will the Queen ask the leader of the majority party to form a new administration.

During the period between dissolution and polling day, however, the Government suspends any advertising campaigns and refrains from major policy decisions or announcements. This was known as 'purdah' but is now usually called the pre-election period.