It may be an instance of rhyming slang, where the second word doesn't keep its meaning. Sometimes instead of saying "OK", I say "okey dokey". If it's rhyming slang, yum scrum would mean something delicious to eat.

I have never heard of "yum scrum" before. A quick search online shows there is a company that makes little cakes called "yum scrum cakes." I know "scrum" has to do with rugby, in certain contexts. I work in software development, and we have a "scrum" each morning, which is a short meeting to revisit the overall goals of a project, decide if anything needs to change, and outline what progress we expect to make.