I use the book people too, we used to have them at work and when I left I looked on their website and ordered directly.
They'll send you a brochure through the post every month, no obligation to buy at all, and quite often there is free postage. I think the postage is reduced anyway if you'e a pensioner and even so the books are very cheap so it still makes it worthwhile to buy.
I just ordered the whole set of 13 hardback Lemony Snickett books for my daughter for Christmas, although they're maybe a bit old yet for your grandchildren.
My daughter loved the famous five books (as I did as a child) and also Roald Dahl storied are good, Jaqueline Wilson is great ...and Dick King Smith ...Stig of the Dump is a nice one too.
We've not had Michael Morpurgo but I did notice the book people had a set of his recently, 12 books for �12.99 here
http://www.thebookpeople.co.uk/webapp/wcs/stor es/servlet/CatalogSearchResultView?storeId=100 01&catalogId=10051&langId=100&pageSize=10&begi nIndex=0&searchScope=SimpleSearch&resultType=2 &resultCatEntryType=1&catgrpSchemaType=2&vipSt atus=&fromPage=basicSearch&searchTerm=morpurgo (hope the link works, if not just go to the site and search on Morpurgo) it says for 10 plus but if she's reading Harry Potter she might be ok .
Hope this helps