But your claim isn't true for 2015 either.
https://en.wikipedia.org/wiki/2015_United_Kingdom_general_el...
36.8 + 12.6 = 49.4%
And there wasn't any election in 2016. Do you mean the referendum? You cannot 'split the vote' in a yes/no referendum, can you?
Your claim is also not true for 2017, before you try that one as well.
https://en.wikipedia.org/wiki/2017_United_Kingdom_general_el...
42.3 + 1.8 = 44.1%