The out-of-the-box ILL functionality does not require any of the request metadata fields to be completed on the form except for the destination library. For example, an article request can be submitted without the article title. I suggest creating a sys pref that would allow the selection of fields that are required before the form can be submitted, based on the request type [Book / Book Chapter / Journal / Journal Article / Conference Proceeding, etc.].
Hi Niamh, I am not sure how to get to that form in Koha without any backend installed - do you maybe refer to the FreeForm backend?
With the standard backend this is an issue we are running into with some of our partners. A "Standard mandatory fields" option would be nice, though the different request types complicate that.