This came up in the course of today's Tolkien Society online pub quiz, in which one round was devoted to questions about the time of day that events happen. (Quick, what time did the auction of Bilbo's estate begin at the end of The Hobbit, and what time did Bilbo arrive home to find it finishing off?)
At Bilbo's Long-Expected Party, we are told that the fireworks started at six-thirty in the evening.
In mid-September, by British Summer Time, this is far too long before sunset at northern latitudes, and the fireworks won't be visible.
If it's GMT, however, that's just about sunset, and the perfect time to start the show.