Aye, I remeber chugging through supposed prospective changes in legislation a couple of years ago with then then NotYetLegallyMrsHob.
It seems marquees are OK because they have a roof. I think the wording was 'roofed structure'. Which made us wonder if the owners of a cave could apply for a license.
But we decided getting married in a cave would be silly. Extremely good fun, but silly.