Once Upon a Time was a very nicely produced educational comic for younger readers much in the tradition of Look & Learn. It's hard to value but a first issue in average condition could fetch £15 and maybe £50 in very top grade,.
They tend to show up on eBay UK in groups rather than single issues, working out to about a couple of pounds each.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.