I may, or may not, be the only one to feel this way, but:
I'm much more likely to enter a few quid in an optional box as part of the check out process (a bit like ebay's) than to go to a dedicated donate via Paypal.
I can't figure out the logic (although I can guess), but there you go.
It is easy enough to add a donation box to any online payment system (just add a "donation" product to the store, priced at £/$/€1, and prompt customers to set the quantity to the amount they wish to donate. Simples! Collecting Gift Aid data for UK taxpayers is just a little more convoluted, but can be done.
Besides, The Foundation uses the Pay Pal guest service to process credit and debit card payments for those who don't have or wish to use Pay Pal accounts, so all payments go through the same channel. Seems logical to me.