Non-tariff barriers are just as real and even more of an obstacle to trade than tariffs. At least with tariffs you can pay the tax. Paris fancies itself as Europe's financial hub. Why would the EU let all of its financial trade go to a country outside the Union? France certainly won't allow Passporting of financial services to London long term, while it aspires to become the fiance capital, and in time it would imo move to Paris because the EU will make it harder or impossible to passport into the EU without a trade agreement. Laws and regulations are massively important and can have a huge impact on the UKs exports to the EU.
Exports of services to the EU are significant: in
2014, the UK exported services worth £84 billion to the EU (37% of all UK exports to the EU). UK imports of services were worth £63 billion (22% of all imports from the EU).
https://fullfact.org/europe/uk-leaving-eu-trade/
Brexit is anti-trade, and anti-prosperity.