Letter to supporters
Although the letter is going to the charity’s supporters, they are people who have signed up to a mailing list. They are not members, regular donors or actively involved in the charity’s work. They are therefore not committed supporters, and the public test is met.
Call to action to voters
There is an explicit call to action to the voting public. However, they are not being called on to vote, they are being called on to lobby politicians. The activity is really aimed at candidates, MPs and ultimately political parties.
Both the email to supporters and the letter to candidates are positive about the policy. They are not positive or negative about parties or candidates.
Context and timing
The charity has campaigned on the issue for several years. Theresa May has committed the Conservatives to the policy. It is reasonable to expect that other larger parties may also add it to their manifestos, but there are some high-profile parties, in particular UKIP, who are known to be against it. While the issue of foreign aid and the 0.7% policy does therefore mark some sort of dividing line between parties, it is not closely enough linked in the public mind for campaigning on the policy to be reasonably regarded as campaigning against UKIP.
How a reasonable person would see the activity
A reasonable person would think that the intention of the activity was to influence political parties and their manifestos. They would not see the campaign as intended to influence voters.
This activity cannot reasonably be regarded as intended to influence voters to vote for a political party or category of candidates, so the purpose test is not met. The spending on the activity is not regulated.