You may consider flipping the script like, “No dad, regular milk is too cheap”* which is true enough, and also a pretty solid comeback. The shamelessly shy over-the-counter-price of milk is enabled by the government paying subsidies to farmers, in order to keep it low. If dad wants to get into a debate over the ties between agriculture and the political powers that be, well, buckle up. More likely though, he will be dazzled by the fact that you used words like “subsidies” and “over-the-counter-price” and give in. BONUS TIP: Offer to pitch in the price difference between oat drink and cow’s milk next time you go shopping. This might technically be a bribe, but you know, the end totally justifies the means.
* UK average farmgate milk price for October 2020 was 29.78ppl (Yeah dad, your offspring knows their shit.)
Defra (2020) United Kingdom milk prices and composition of milk statistics notice (data for October 2020)