Hey lewis,
Good Polarizing filter for the daytime cloud stuff is great. If you want a stab at daytime lightning without a trigger the ND8 which I know you have would be great. I've never done this successfully myself.
Almost everything electrical you bring over there will be rated for 110 - 240 v (your camera(s), camcorder,laptop, phone etc... check the power supply first though to be sure) so what most of us bring is a 4 gang (or more) UK extension lead with a 1 US to UK converter so you can charge 4 or more devices without hogging every socket in your hotel room.
See ya soon.