I make my DH do my shopping in the UK and then bring it out to France. I only buy my veg here at the market and a little bit of meat. Fish is much cheaper though, but I think that's because we only live 30-40 mins from the coast and the French do like their fish. It's funny you should mention cereal that's what we buy in cereal mountains, boxes and boxes of the stuff. Boxes of cereal are half the size and twice the price in France and my DC's will have two or three bowls every morning before school.
DH and I are very careful with money at the moment with the price of things going up in both countries so we pretty much compare prices on most things before we buy, even a packet of lentils in the local supermarket in France is double the price of a larger packet in Asda. Soap powder etc all cheaper in the UK.
Even unleaded is cheaper in the UK, although I think diesel is still cheaper in France.
My electricity in France has gone up 60 euros a month in the last 12 months and I use less now than I ever have.
When we first bought our house and it was a holiday home we would stock up on clothes for the DC's in France and take back food etc to the UK, it is definitely very much the other way round now. Even buying things for renovating the house; insulation in B&Q is half the price that you can buy it here, and the Brico shops here are run by Kingfisher Group too so it's the same company.
However FC I think there is now a company operating in France who will bring you your shopping from the UK for a percentage of the price of the shopping and it's still working out cheaper than using the French supermarkets. I've noticed a lot of people on the Brittany forums are talking about it, I'll have a look and see if I can get the details for you. I think they'll even bring you stuff from Argos and M&S.
I don't deny that prices have gone up in the UK but we are a family with a foot in both countries so to speak as DH is living at our place just outside London full time and we're living in France full time so I think I've got a pretty good grasp on the differences in both countries. What always gets me about France is that it seems so non competitive with things like mobile contracts, broadband etc. My French mobile costs twice as much as my DH's UK phone and I can't afford to make any calls on mine, whereas he can't use all the free stuff he's given with his.
Still we're here now and in the summer it will be lovely and I'll remember why I love living in France, winters are always grim I find.