Assuming the UK could get itself together and find the political willpower to do so…it still won’t be easy. The EU has to agree to it, and it would require all members of the EU approving them rejoining.
As I recall, the EU warned them that if they left it would be very challenging for them to rejoin. The idea was to discourage them from leaving the EU in the first place. But they did. And now they have to live with the consequences.