I think that you would be OK to issue an invoice. However my understanding is that there is no actual contract in place with the charity, I guess it is up to them how this is actually shown but if you have issued an invoice then your records will be correct :)
Just looked this up on Wikipedia...."An honorarium is an ex gratia payment (i.e., a payment made without the giver recognizing themselves as having any liability or legal obligation) made to a person for their services in a volunteer capacity or for services for which fees are not traditionally required. This is used by groups such as schools or sporting clubs to pay coaches for their costs.[1][2] Another example includes the payment to a guest speaker at a conference to cover their travel, accommodation, or preparation time."
