In searching for Summer why don't Tom and Lily return to the cottage to pick up Lily's wedding dress?

Earlier in the story, Mr. Noakes says "If I could find a bit of sunshine near here, permanent bit that is, dja know what I'd do?"

The young people looked at him inquiringly across the bread and margarine.

"Lido, trailer site, country club, holiday camp-- you wouldn't know the place."

So the answer to your question is--- they knew that if they were to go back to her house, where the sun was, then Mr. Noakes would find it. And Tom reminds Lilly at the end of the Story at she wont be needing it again.