Okay, I called CAF to ask about this, as I was interested too. Apparently the Charity Account's 4% fee is a one-off at the time of donation, rather than annually recurring. You can hold the money indefinitely after it's put into the account for no fee. The fee is 4% on the first £22.5k donated per year, after that it reduces to 1%. (They linked me to a PDF detailing this here.) So this seems like it could well be worth it, in cases where you have more than a few £000 to donate to registered charities, if it would be more efficient to be able to move across tax year boundaries?

They also offer something called a CAF Charitable Trust, which is a fuller-service Donor-Advised Fund (not in fact a charitable trust...). It has a minimum opening balance of £10k, and this one (a) does have annual fees, of 1.2% to a minimum of £120/year, then reducing for very high balances (over £100k); and (b) allows investment. Their investment options seem pretty good: their basic service lets you access a range of funds called the "FP CAF investment funds" – the link was broken to on their website, but I got the impression there was a reasonable range of them, and the person on the phone said she'd email me details (I'll edit when I have them). They also have a "Premier service" for balances over £25k, with a higher fee of 1.5%, which lets you invest in any UK-listed fund or ETF that fulfils some requirements (which in practice are apparently usually fine).

They also said that, with either the Charity Account or the Charitable Trust, you can make donations to any UK-registered charity, most overseas registered charities, and sometimes social enterprises (overseas charities and social enterprises have an approval process to go through, which they're willing to do for a large enough donation).

I think I may open one of these at some point in the next couple of years, will update if I do!

My impression from CAF's webpage on their Charity Accounts was that the 4% fee was a one-off when you contribute money to the account, rather than an annual fee on the balance. However it's not very clear and the other interpretation definitely makes sense too. Is anyone's knowledge from a source other than the website?