Great question!

One can claim Gift Aid on a donation to the Patient Philanthropy Fund (PPF), e.g. if donating through Giving What We Can. So a basic rate taxpayer gets a 25% "return" on the initial donation (via gift aid). The fund can then be expected to make a financial return equivalent to an index fund (~10% p.a for e.g. S&P 500). 

So, if you buy the claim that your expected impact will be 9x larger in 10 years than today, then a £1,000 donation today will have an expected (mean) impact of £11,250, for longtermist causes (£1,000 * 1.25 * 9)^[1]

Therefore I think the question of:

 "donate now and claim gift aid" OR "invest then donate later"

...can be reframed as: 

"donate now and claim gift aid" OR "donate to (e.g. PPF) now and claim gift aid, for the PPF to invest and then donate later"

 (I.e. I think gift aid considerations don't favour one option over the other)

Of course, one may reasonably disagree on giving now vs giving later - this is a much more messy question, and one that I won't attempt to answer here.

I'm not sure about paying into an organisation's fund.

I think that conditional on giving later, the PPF is a better option than individually taking an "investing to give" approach (roughly for reasons described here)

(disclaimer: I work on the operations side of the PPF)

1. ^{^}

   A £1,000 donation becomes $1,250 for a basic rate taxpayer. Over 10 years, expected impact will increase by 9x (using the Investing to Give report model's mean estimate)

   Using the same logic for global health or animal welfare, your expected (mean) impact from a £1,000 donation in 10 years would be £2,625 (£1,000 * 1.25 * 2.1x) and £5,250 (£1,000 * 1.25 * 4.2x).

   Note however that no "PPF equivalent" for global health or animal welfare currently exists, AFAIK

Thanks for raising this question!

One consideration I'd add that I don't yet see reflected in your post or in the comments is that it could very well be that there will still be some sort of tax relief when you give later (and that it could even be larger at that point!), so tax relief may inform the giving now vs giving later question less than it may seem at first sight.

It's possible there is reasonable case to be made that tax relief on money donated later is going to be (much) lower in expectation than on money donated now (e.g. perhaps the current system priviliges small yearly donations over large single ones, or perhaps there are longer-term downward trends in policy around tax relief), but that case needs to be made in order to make the claim that current tax relief systems push (strongly) in the direction of giving now.

(I don't know much about this at all, but FWIW my best guess would be tax relief for an individual giving later will be slightly lower in expectation than for an individual giving now, which would push slightly - but only slightly! - in the direction of giving now)

Very interesting question!

I don't have that much to add, it seems like you've got a pretty good handle on the different options available. Though a couple of things:

• That DAF post you linked to is a bit out of date (I keep meaning to write an updated version!), since the CAF DAF now charges 1.2-1.5% on balances up to £100,000 (and less above that), with a minimum balance of £10,000-£25,000, so is a more viable option even if you aren't a mega-donor
• ISAs are still a ridiculous vehicle, you can put £20,000 in per year, and be exempt from capital gains, stamp duty, dividend tax etc forever on that money (today is the last day of the tax year so fill your ISA today for this year's allowance!). Though you do contribute from your post-tax income.
   

I would say you should just donate it now. Gift Aid is just very efficient, and we have plenty of effective interventions in all these areas to do now.

For x- and s-risk and global development (areas that benefit from research and can accumulate knowledge) the time of highest leverage is plausibly now, and the earlier the better.

The report you link says that the largest cause of later cost-effectiveness is "exogenous learning", i.e. research that happens regardless and makes marginal interventions more effective. If that is the case, why not invest in the learning itself, to make effective interventions for everyone appear closer in time?

I'm less sure about animal welfare. Research affects it somewhat but it's mostly advocacy, and I'm not sure how that accumulates over time. I think you should give now too.