Previously, cash deposits made into a charity collection box would offer no tax refund to the charity. This has now changed; a cash donation of no more than £20 is treated as a gift, after deduction of basic rate tax, which the charity can now reclaim.
This is subject to a modest overall limit to the charity of generally £5,000 a year.
Within this limit, therefore, a cash donation is just as valuable to a charity as a payment through Gift Aid, a cheque, a direct debit or a bank transfer. If you pay tax at more than the basic rate, there is no higher rate tax relief due to you on a cash donation. If this kind of relief is what you wish for, you need to use Gift Aid as before.