Early issues (#1 to #20) are collectible and don't often show up. Titles like this along with Eagle tend to be sold in volumes rather than single issues. Less collectible than Eagle but still worth picking up and reading. The last 30 issues or so are collectible for the fabulous full front cover aircraft illustrations.
Collectible characters to watch for: Tarna the Jungle Boy, Michael Bentine's TV Bumblies and especially King Arthur and His Knights by Frank Belllamy and Robin Hood by Frank Bellamy.
With thanks to noted collector Steven Adamson for some great free gift pictures.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.