Payroll giving
Payroll Giving is a government scheme to make it easy for employees to give to charity. If you choose to give to the Unicorn through Payroll Giving, the payment is made before tax, which means that a gift of £10 will only actually cost you £8 if you are a standard-rate tax payer (or £6 if you are a higher rate tax payer).
Joining the scheme is easy to do, please ask your employer about Payroll Giving.