The main reason Leave won is because there was strong popular demand for "control of immigration", which implies leaving the Single Market. That doesn't mean the government won't quietly disregard that, but it does mean it's considerably less likely we take a Norway-like approach than it would have been if the public was instead obsessed with minutae like the Common Fisheries Policy or European Court of Justice.

Immigration restrictions are the least of worries for UK-based startups not focusing on the domestic market though, especially in fintech which has traditionally been London's strength. If you need company registrations, offices, contract and potentially even servers and compliance staff to be based in the EU to comply with certain regulations, it suddenly makes a lot less sense to spend all the extra overheads of having your [main] office in London, especially if other startups on the scene are starting to migrate and you don't want multiple offices. And whilst EU negotiators might want to avoid tariffs regardless of whether the UK stays in the Single Market or not, they have very little reason to secure trade agreements with the UK that grant UK firms passporting rights.