We get ours from Wilko's. They also have some in Poundland but I don't know how good they are.
WH Smith sells the slightly more expensive Carol Vorderman workbooks. I didn't want to like them but they turned out to be pretty good, they were laid out really well and explained the theory behind each exercise.
My pre-schooler ( just turned 4) sometimes gets the Friends comic as he likes their workbook. He also likes the Parragon Gold Stars workbooks. He's got a wipe clean Priddy book called Let's Get Ready for School, and whatever that cost it's been worth it as he gets that out a lot. I got that on Amazon.
I sound like I am hot housing him - I am not. He just loves shapes and numbers and sees his older sibling doing school work so he wants to join in. And to be honest he has co-ordinate difficulties and I think him sitting down with a pencil will help him.
I find the Cbeebies magazines quite good actually, helps that he likes the characters and wants to the activities based on them. I too have the Gold Star workbooks and also wipe clean ones (We buy from a discount book shop and costs about £1), he likes them all. He has only recently taken an interest in writing (will be 4 in 3 days), but likes these ones. He also likes join the dots books.