The next Scottish Parliament, Welsh Senedd, London mayor and London Assembly will be determined. Sir Keir Starmer risks a major blow if the Conservatives succeed in winning the by-election for Hartlepool, a traditional Labour stronghold.
Elections have also taken place for 13 regional mayors, 39 police and crime commissioners and 143 English councils.
Counting will begin overnight in some areas with results expected from Hartlepool and certain council elections.
However due to the coronavirus pandemic counting is expected to take longer than usual, with most results coming in between Friday and Monday.
London mayor Sadiq Khan is hoping to secure re-election against Tory rival Shaun Bailey.
9.35pm update: Sadiq Khan's election nightmare - Low London turnout sparks panic
Labour's internal numbers show in-person turnout for the London Mayoral election is far lower than in 2016, raising concerns within the party's ranks.
Figures suggest just 11.6 percent of the total turnout at the last London Mayoral election had turned up at polling stations by midday.
The figure is far lower than had been expected by the Labour Party, driving concerns of a shock result.
In Scotland the SNP is seeking the mandate for another independence referendum.
Unionist parties are aiming to weaken Nicola Sturgeon by depriving the SNP of an absolute majority.
Welsh 16 and 17-year-olds were able to vote for the first time for the Senedd election.
Labour is expected to remain the dominant party in Wales but is facing challenges from the Conservatives and Welsh nationalists.
Hartlepool has always voted Labour since the Parliamentary constituency was created in 1974.
A Conservative win here would be a huge boost for Mr Johnson and a devastating blow for Sir Keir Starmer.
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