Many of our foodmarkets have 'charity' bins at the checkouts, for pet and human goods to be utilised locally not by some national organisation, they are always full. Do you have that set up in the UK?
I wouldn't donate to the RSPCA full stop.
I'd rather go and buy a sack of food and give it to a shelter in person. That way I would know where my money was going.
What gets me is why can shelters not use out of date feed, even by a few days? One of my Boarders works in a pet store and supplies me with animal food just a few days out of date, she says that the shelters can't/won't take it! Stupid.