Why have I been charged for my roaming data?

In the EU you will be charged for roaming if you exceed your allocated allowance of 16GB.

Data allowances if available at the time of roaming can be used while roaming in the EEA subject to fair use limits where applicable. EU roaming fair use allowance is calculated in line with Regulation (EU) No 531/2012 (as amended) and Data wholesale roaming price caps. Where the domestic data allowance has not been fully consumed but the fair use allowance has been fully consumed, a surcharge rate of €1.91 per GB/€0.0019 per MB (based on Regulated rate of €1.55 per GB excl. VAT) will apply until the domestic allowance has been fully consumed. Standard domestic rates apply thereafter.