What would be a good 1-2 page short story to use to help students see the relationship between good and evil as a comparison to Lord of the Flies?
Wow! One to two pages is a tough one. I can't think of any short stories that are that short. Some of Hawthorne's stories would work - Young Goodman Brown or The Minister's Black Veil, but neither are short. Some Sherlock Holmes tales would work but again, they are not short. When I taught this novel, I used excerpts from Screwtape Letters - even though it is a book, you can find short exerpts in it. Also, selections from Mere Christianity are good for discussing the nature of good and evil, although neither is short and the latter is non-fiction.
If you are teaching seniors (I am assuming, since it is British Lit, no?), you can use some of Stephen King's short stories. I don't know of any that are only 1-2 pages, but some of them are pretty good (The Thing at the Bottom of the Well, The Cursed Expedition) but a word of warning - they can be pretty gruesome and perhaps not appropriate in all instances.
How about some Bible stories? You could use the story of Jezebel and Ahab, the story of Joseph being sold into Egypt by his brothers, the story of Job (classic conflict of good vs evil), the story of Cain and Abel, etc.