One could argue that, due to its sheer size, food security is less of an issue and the EU could do with less agricultural subsidies (not sure I'd agree but someone could make that argument). That is for the whole 28 countries, half a billion people strong EU.
If anything, the UK urgently needs more agricultural subsidies for at least a couple of years. 40% of all our food is imported and the UK is looking at going from trade agreements with over 60 countries to zero. Both food imports and exports are going to be seriously affected for a couple of years so subsidising farmers to make the UK more self sufficient could be of strategic importance.
That means that it helps to know who produces what (and who is fiddling what to claim subsidies they should not receive). That means we'll need a system to keep track of how many hectolitres of milk the cows of farmer X produce and how many kilos of sheep meat farmer Y produces on an annual basis.