I reckon we will feel the chill at 10:00. Those starting from Hebden Bridge at 08:00, especially so because the valleys are slow to warm up.
It is forecast to get a lot milder later on, but the easterly wind will probably stop it feeling quite as warm as it is.
Layers! Wear some extra kit which you can easily take off, like arm and leg warmers. You will probably keep it on all day, but it is nice to have the option to take it off. I don't think a heavy duty winter jacket would be a good idea because you might get too warm wearing it, but too cold if you took it off, and it would be bulky to carry if you did.
My toes got cold on my ride yesterday. I don't think I will want to wear my heavy neoprene overshoes so I might just put some duct tape over the front of my shoes to keep the wind out!