In a worst-case scenario, that could leave Brits reduced to the status of Americans, who face charges of up to €50 for each song they stream or €10 per MB of data whenever they visit Europe.
The EU will abolish roaming fees for all its citizens in June 2017, but that won't apply to countries outside the EU. And the EU cannot cut a copy-cat deal with a non-EU country because WTO rules require the same deal to be offered to all 164 WTO countries, the FT says:
"A quick bilateral deal between the UK and the EU to cover roaming is not possible, according to the European Commission. In a response to a question on the topic last year, Günther Oettinger, the German commissioner responsible for the bloc's telecoms policy, said that "there are obvious constraints".
"Under WTO rules, any bilateral agreement outside of a comprehensive free-trade deal would have to be extended to all other WTO members, warned Mr Oettinger in a response to the European Parliament last year."
The EU has been particularly aggressive on behalf of consumers. It has spent the last 10 years creating the agreement to kill pan-Europe roaming charges. Post-Brexit, Britain is likely to find itself in the same position as Switzerland. QZ reports:
"As the think-tank Bruegel has pointed out, because Switzerland is not part of the EU, the Swiss currently pay one of the highest rates in Europe for data roaming. "It appears that the UK will shortly find itself in the same position," Bruegel senior fellow J. Scott Marcus wrote in June."