I'm seeing a few stores here and there which have a "round up to donate" option. I guess I'm a bit of a sucker and I always use that option. My groceries are always a round number as a result.
Just set up a direct debit to your favourite charity.
the point of these drives is to get more people to give to charity. Then you use a lullaby-word as if setting up a charitable donation is as easy as saying "yes" when the checker asks if you want to give a small donation.
Its actually very easy to give £5/month direct to a charity. Takes about 2 minutes, just gotta do it.