I am thinking of the Oct. 19 sailing myself..the murder mystery theme should be a hoot..
I would bring layers..light sweaters/sweater sets, jackets etc..I have been in London in Oct and it is cool and damp..It can get windy on deck and if it is cool the wind chill will make it feel colder but the ship has lots of cosy nooks and blankets!!
But look at the crazy weather this spring..I live on eastern Long Island and have yet to wear shorts let alone go to the beach...so travel prepared for anything....
Well... I've done it in mid/late September and October... a few times... it can be "bracing" on deck, but its wonderful. No one can predict, with any degree of accuracy, the weather on the North Atlantic at any time... which is part of the fun of the crossing, I'd say.
For much of the last Century and the Century before, it was the only way to cross... regardless of the season. There is so much of interest going on all the time during a crossing that you'll probably not be spending too much time on deck. But, regardless of the weather, it is hard to beat a few turns around that classic ship's Boat Deck.