Best power bar for my situation?

I'm going to London for two days and am planning on taking quite a lot of pictures using my iPhone 6. I'm looking for a power bar that will give my phone more than 2/3 charges, but will also not take up too much room in my bag, and not be too heavy. Can you suggest anything that will fit these specifications?

We have electricity in London, you know. You can charge your phone in your hotel room.

There are hundreds of power bars. Shop around a bit.