The official briefing says:
Where there is a tie between two or more candidates receiving the same number of votes the Acting Returning Officer will decide the result by lot. It is a matter for the Acting Returning Officer to determine the method used.
Normally this is a coin toss or drawing names from a hat.
Sadly, “by lot” rules out more inventive suggestions which many of us here at the New Statesman would like to see instituted, such as an arm wrestle or a deciding game of Connect Four.
Interestingly, at point the Returning Officer at the count was allowed to award an extra casting vote. This happened at an 1831 Bandon by-election.
Other types of election also use different methods. The European Parliament Presidential election goes through several stages, but if no-one is declared a winner by the fourth ballot, the older candidate is declared President.
In the US presidential collection, a tie – where the Electoral College is split 269 to 269 – the House of Representatives would have to select the president through their own vote.