In the event of a deal, surcharge-free roaming would continue to be guaranteed during the Implementation Period. Following the Implementation Period the arrangements for roaming, including surcharges, would depend on the outcome of the negotiations on the Future Economic Partnership. The Political Declaration on the UK's Future Economic Partnership proposes a framework for negotiations with the EU, including for services and digital sectors. That approach would not preclude discussions with the EU on arrangements for consumers, for example in the area of mobile roaming.
In the event of no deal, the government has published a technical notice on mobile roaming. This is available here: https://www.gov.uk/government/publications/mobile-roaming-if-theres-no-brexit-deal/mobile-roaming-if-theres-no-brexit-deal