If you're big fish eaters definitely Morrisons, they have an excellent fish counter and have recently bought a Cornish fish supplier who couldn't export their catch due to Brexit. Also look at Iceland if there's one near you, they sell far more than frozen pizzas, chips and beige food.
Also look at greengrocers or markets for the fruit and veg, or the wonky boxes. Try to shop seasonally as this will reduce the cost.
But it doesn't have to be the same supermarket every week or the same produce every week. Nothing to stop you going to Asda one week, Morrisons the next etc etc.
Definitely a family meeting to explain how they've been extremely priveliged up to now and previous unlimited fresh blueberries are sadly unaffordable from now on. But frozen blueberries, raspberries etc are much cheaper and fine for cereal or in yogurts or muffins etc.
Might have to consider changing your diet to adjust to your new budget - look at frozen and canned fish as cheaper than fresh. Also incorporate a few vegetarian meals each week, chillis and curries etc with lots of pulses and seasonal vegetables are very cheap and healthy - Morrisons has a good world food section where you can get packets of spices for about 50-70 p that have about 3 times what you get in jars, you could set yourself up with a good range of spices for under a fiver that will do a few months worth of meals.
And you could try growing your own, raspberries especially can grow very well and in season, you could send the DC out to pick them off the bushes if they want them.
It's probably also worth looking at your overall budget, so you know how much you can afford, make the best of your new circumstances and to make sure all your expenses are covered, while hopefully freeing up some money for the occasional treat. Have a look at:
www.moneysavingexpert.com/family/money-help/
Good luck!