Incorporating a startup in the UK becomes much less attractive after Brexit because EU markets are not easily accessible anymore. I think EU founders will take into account this first. If you add paperwork on top of that I don't see why founders wouldn't just incorporate let's say in Estonia and just travel to London on a tourist Visa to look for funding if needed.
Catalans voted in a referendum a new regional constitution which was overturned by the Spanish constitutional court. This triggered support for independence. Catalans don't manage, their taxes, nor their airports, or their trains. Everything that really matters is managed by the central government, it's self-rule on paper but not in reality.