There might be a deal. There might not be. Either way, the FCA has done quite a lot of work in preparation for BREXIT and you can access that information here.
The information includes a guide for firms in preparing for BREXIT.
The FCA has made arrangements for non-UK firms to continue operating post-BREXIT. Unfortunately, to date, the EU has not reciprocated and UK firms that currently passport services into an EU or EEA country, or who wish to do so, will need to contact the relevant regulator in the countries where services are provided to identify what is required to be able to provide those services after BREXIT at the end of March 2019.
The EU/EEA countries are Austria, Belgium, Bulgaria, Croatia, Cyprus (Republic of), Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden.
Details of all regulators in the EU/EEA can be found here.