At present, under the CFP’s principle of ‘Relative Stability’, the UK receives a fixed share of fishing opportunities based on historical fishing patterns in 1973-1978. On average, between 2012 and 2016, UK vessels landed approximately 90,000 tonnes103 of fish each year caught in other Member States’ waters, and other Member States’ vessels caught in the region of 760,000 tonnes104 of fish each year in UK waters.
The UK will seek to move from relative stability towards a more scientific method for informing future Total Allowable Catch (TAC) shares.

That would probably be zonal attachment allocation, the spatial distribution of the stock over time and over its various life stages. Which would result in a massive decrease in the EU-27 quota but still allow access to UK waters.