This policy was not polled explicitly and it is impossible to estimate exactly how much support this cost them (or even gained them), but it is worth noting that the UK has some of the most intrusive mass surveillance legislations and programs in the world.
But there is a 2016 Ofcom report relating to adult attitudes towards the media these findings were made
A majority of internet users agree (52% strongly, 26% slightly) that the internet needs to be regulated, in terms of what can be shown and written online, and one in ten adults believe that ‘all’ of what can be seen or read online is regulated.
As in 2014, there is no consensus among internet users as to whether users should be free to say and do what they want online; with younger users and DEs more likely to agree with this.
This is indicative that generally the government are unlikely to lose much support by passing internet regulation as the majority believe more legislation in necessary, even if the particular regulation would be ineffective at combatting terror simply because the public are unlikely to be "tech-savvy" like you and understand exactly what each piece of regulation does.
The second quote is also indicative, since one of the key voter demographics for conservatives are older people. Who tend to believe more internet regulation is necessary, and who often don't believe in unlimited free speech on the internet.