Ironically, charities are only allowed to put so much ink in the pen as it is a "tool" in order for you to "fill in the form". If they put even slightly more ink in them, it is a gift, and therefore taxable. VAT rules, again.
Charities are in a bind, they can't NOT write to you to raise funds, as that would make them delinquent in their charitable aims and they'd raise less money. Likewise, they can't just ASK you for money, as that means they pay more VAT (it's considered a "sale" in the tax rules), and therefore they lose 20% of the money they DO raise, so they are actually forced to write to you, NOT to ask you to give them money, but to make you think about the charity and consider giving them money, as this isn't a sale. The "gifts" in the envelope have to be there for tax purposes.
It's hellishly complicated, and charities are forced to spend money in order to raise money - it makes no sense from the outside, but the alternative is less money for charities, either because people don't know about them (because people get upset when charities try to let people know they exist), or less money through taxation (because the government forces them to do seemingly irrational things to save money).
I get very angry when people say they "refuse to give money" to charities who spend money on administration and fundraising - there are far better reasons not to give money to individual charities, but not giving money because they are forced to spend money just on existing? How stupid.