So, if/when Brexit happens in March, the January transfer window will be the last one for the UK clubs under current rules. While the exact details might change a bit, the biggest guidelines are probably set by now.
"Clubs are currently free to sign any players from countries in the European Economic Area. But strict rules govern the players they are able to sign from outside the EEA. Essentially, they must appear regularly for a country in the top 60 of the world rankings, or command a transfer fee or wages exceeding the median paid by Premier League clubs the previous year, to suggest they are an exceptional talent.
The Premier League fears that these rules could now be extended to all European players, obstructing clubs’ ability to recruit from the continent."
"Overall, the majority of European players who have joined clubs in Premier League history - 591 of the 1,022 players with EEA passports who signed from 1992 until the end of the 2017-18 season - would not have qualified for a work permit when they were signed"
Why Premier League fears work permit changes after Brexit could make another Leicester miracle impossible
So apart from the the effect on quality of teams/clubs, the financial aspect for clubs all over Europe will be huge, if half of the transfer fees from premier league clubs to continental clubs is wiped out.
Although PL has asked the government to abolish all work permit criteria, but hard to believe that happening.