I Would suspect at least 250,000 Ballot Papers were "Spoilt", perhaps as much as 3000,000 Many on Purpose, as a Protest.
In the 2010 General Election, the UK total of protest and 'spoilt' votes was around 295,000. That is a large number, but it represents only 1% of voters. Meanwhile, 34% of registered voters (16 Million) just didn't vote. If just some of the unheard 34% Vote for what they believe in, the results of the election can change.