As it stands today the exchange rate for the Euro sits at €1=£0.83333, a change of 13.9% from the €1 = £0.73129 rate set by the European Central Bank in September 2015 that converts CAP support payments into £ sterling. If the £ sits at the same 83p per Euro level then in the short term UK farmers will see and equivalent boost in BPS payments in December 2016.
It doesn’t stop there, producers who rely on an EU export market for their product will find that export demand will increase since the cost of purchasing from abroad will be less. Lamb falls into this category (75,338 t to EU in 2015) (AHDB data) however it will take an additional 40,000 t exports to take it back to 2013 levels. In the 4 months to April 2016 there is little evidence of additional appetite for UK lamb on the continent and it will be interesting to see if export trade increases significantly post the Brexit vote and the collapse in the £.
The long game is different, trading relationships are uncertain and the CAP will disappear from the UK in 2019-20, certainly before the onset of the next funding period. There appear to be short term income gains for UK agriculture but long term its much less certain.