You'll probably use two or three batteries shooting per day which you'll want to recharge at night. A single battery charger will be a pain as you'll have to keep swapping the batteries out to charge them all.
The VPB charger will charge the two batteries in the grip quickly and simultaneously. You can use a USB charger and cable to charge the in camera battery at the same time without removing it from the camera. The USB cable will also give you an emergency charging option should your main charger fail on the trip.
Here's a generic tip for traveling abroad.
Try to pick up a really compact 4 socket power strip with a really short lead (to keep it compact and light). You then only need one US to UK adapter to allow you to plug in all your appliances in your hotel room. You might even be able to find one with USB outlets so you don't have to carry a USB charger at all.
