While you are in the UK, you would presumably be on a data plan, so yes, it would use a small amount of data if you are out. Otherwise, it would use your WiFi connection. You do need to have a data allowance, as otherwise, you will be quite limited as to use while away from your WiFi. But it needn't be huge. I am on my phone quite a lot each day, and I never exceed 1Gb of data per month, even though I have unlimited data with my legacy O2 contract.