The Roman Mysteries were done on the BBC and I have the DVDs of them, so I wouldn't count them.
If I was rich I'd like to have a film company that focuses on doing good adaptions from children's books in keeping with the spirit of the book. Yes, some changes do need to be made, but keeping the characters consistent with the original book (yes, I'm looking at the recent Swallows and Amazon film, Little White Horse and the dreadful attempt of The Dark is rising-the latter has a quite reasonable BBC radio version up now though)
These would be the books I'd start with:
Malcolm Saville: Mystery at Witchend (and maybe other of the Lone Pine series)
Monica Edwards: The White Riders (would be very cinematic), Cargo of Horses, Storm Ahead and maybe more.
Diana Wynne Jones: The ogre downstairs (would be brilliant filmed), Chrestomanci series (less sure on that, not sure it'd work on screen) and maybe The Year of the Ghost and Power of Three
Arthur Ransome: Winter Holiday, Peter Duck
Enid Blyton: Find Outer series. The Secret island (don't think that one's been filmed)
Elinor Brent Dyer: First Few Chalet School books ending with them setting up in England after fleeing Guernsey.
Violet Needham: Possibly the main Stormy Petral lot, but definitely-The Black Riders, The Stormy Petral, The House of the Peladin, The Emerald Crown
Antonia Forest: Maybe the school ones, but I think The Marlows and the Traitors would film well, and I'd like the historical ones-The Player's Boy and The Player and the Rebels too.
Gwendoline Courtney: A Coronet for Cathy
Geoffrey Trease: Bannermere series and Cue for Treason
Noel Streatfeild: The House in Cornwall, The Children of Primrose Lane, Tennis Shoes, Party Frock, White Boots (last 3 under their original names please), The Fearless treasure (possibly done as a history programme)
Possibly:
Jane Shaw's Susan series might film well
Elizabeth Goudge: Island Magic, Linets and Valerians, Smokey House
Joan Aiken: (Wolves was filmed, not particularly well, if I remember rightly) but maybe others from the Dido series, or the go Saddle the Sea trilogy.
Those are a older selection of books, but I think a lot of them would film well. Some could be updated to be more modern, but others were benefit from being set in the original publishing .