I've been happy with Zooplus so far, been using them on and off for about 18 months. I like their prices, they are normally the cheapest for the dry dog food I buy and if not, only a pound or two more expensive for a large bag. Delivery is free and quick.
The second time I placed an order I added an incorrect bag of dog food by mistake. When it arrived I informed them and they got the courier to pick it up the next day at no cost to me and at the same time delivered the item I meant to order, was very impressed.
Another time they sent an incorrect item, was a glow in the dark collar instead of a dog toy. They let me keep the collar and refunded me for the toy so was satisfied with that too.
I don't buy my dry dog food from anywhere else now and usually add a few extra odds and ends in to the order at the same time