Difference between this and normal canvassing
In normal canvassing, the canvasser identifies themselves as such; if you answer them, then the DPA does not require them to clarify what they are using the data for - it's assumed to be implicit. The problem with this app is that the "canvasser" is your friend or work colleague; when you tell them how you're voting, you see that as a personal conversation, not giving data to the Tory party.
Incidentally, re: TPS, if the person only asks you how you will vote that's OK; if they try to persuade you to vote for them, it isn't. Sensible political parties have two scripts for their canvassers, one for TPS-registered people that tells them not to try to persuade, one for non-TPS registered.
Finally, advice for anyone who wants to be left alone by canvassers:
1. Find out which party your current MP is and who came second last time.
2. If you are called on by any party other than your current MP's party, say you are voting for your current MP. They will leave you alone in the hope that by them not mentioning the election you will forget to vote.
3. If your current MP is Labour or Tory, if you are called on by their party, say you are voting for the opposite party. Again, they will leave you alone.
4. If your current MP is not Labour or Tory and you are called on by their party, say you are voting for whichever party came second last time.
There are a (very) small number of exceptions, mostly three-way marginals where the incumbent is Labour with a very small majority; there is no option there that will easily get rid of a Lib Dem canvasser (well, apart from telling them to go away, but another one will probably turn up in a few days if you do that).