This policy, as opposed to a general duty increase, is targeted at low cost/high alcohol products which, as I understand it, are most associated with anti-social and health consequences, which are issues here. In Scotland it is the case already that alcohol can't be sold before 10am, which was another measure intended to curb the impulse purchase of cheap booze.
Looking at the beer and wine in our fridge right now the cost will be unchanged (if 50p per unit is the figure used) since it already costs more than that, and although I can't stand cider it looks as if the cider brands that are comparable in price to the likes of cans/bottles of, say, Guinness, Sol or Stella etc are already more expensive than what the minimum unit price would be based on alcohol content.
There is a problem here in Scotland with cheap booze and its various consequences and that is what this policy is about - I support it since I think it will, over time, improve matters where I live.