Looking at the c.v.s of some of the players involved in the current Leicester v Glamorgan match, I was reminded of the means by which (especially) South Africans were enabled to be signed to England and Wales counties. The Kolpak ruling means that any player from a country which has a trading agreement with the EU has a right to ply his trade in an EU member state, regardless of any local league rules aimed at preventing foreign domination of teams.
There were over 60 Kolpak players in English cricket in 2008. Measures were introduced then and in the next year to stem the flow, but it is not clear how far these have been effective since there is no readily available list of those overseas players who are here under Kolpak rules and those who qualify under the government's point-based system. It is probably safe to say that, if EU membership is revoked, some counties will be scrambling to move people from the former to the latter category, and that the EWCB on the other hand will be in no hurry to meet their demands for GBEs.
In my estimation, the four-day game is going to be less affected by Brexit than by the sad decline in financial support for it. What will clearly be hit is the plan for a city-region-based Indian Cricket League style twenty-over competition, which must have overseas stars in order to succeed.