I work in fundraising and the best way to donate is to pay it direct to the charity, ideally online to minimise processing costs/staff time. CAF Bank is good too.
There are a few different models for customer donations via corporations.
A) Schmooze model: Named charity of the month/year at the till. Gets a cheque at the end of the period. It usually takes a lot of networking and staff time from the charity to get nominated. Not an open process.
B) Tiny grant model: Consumer gets to choose 1 of X amount of charities, the charities get £500/1000/1500 depending on amounts of plastic coins/app choice. Charities have to apply for this process, it's considered a grant but you have no idea how much you may get in return. Think Tesco, Co-op.
C) The tax avoiding model: Company sets up their own grant-giving charity. Your donation becomes a donation from the company to their own charity. The charity may or may not donate much to actual charities. Often charities have to apply for grants, this may be an open or closed process with lots of foggy unpublished details. E.g. how much they'll give, how often they give, application open and closing dates, how to apply at all. Big barrier is usually needing an employee to nominate you. This is usually office staff, not shop floor staff because they're not even told about it or have better things to do.
D) The tax avoiding PayPal model: Company sets up its own charity. Your donation becomes a donation from the company to their own charity. FakeCharity pays out the exact amount to chosen charity after 1-3 months. Charities have to register to be on the list of available charities to donate to, which is a pretty complicated and long process when they only get a tenner a year if they're lucky. Open process but a hassle.
Edit:
E) The 'it's a con' model. Company sets up it's own charity. Charity does whatever the hell it likes. Gatwick's nominated charity is the Gatwick Travelcare charity 😂 They're using it to pay to "help over 2,000 passengers with travel-related problems each year" like passport and missed flights... You know, the kind of thing that the airport should pay for!