Traditionally dessert grapes seldom ripen well or reliably in England, even the most modern varieties (unavailable in 1941) cannot be relied upon today. Dessert grapes are all but unknown as a UK commercial crop to this day, I imagine the only growers back then would have been vanity growers like stately homes - and perhaps one or two specialists, all in big greenhouses. Few if any of those would have come onto the market.
There was plenty of merchant shipping trade throughout the war so it's quite possible some enterprising owners/skippers filled up corners of their holds with high-value goods. Sailors have always taken advantage of such opportunities, and at over a pound per bunch this would have been a winner I suspect.