Okay, your partner is a food snob, I get that, but you need to have a conversation about how her high food standards is affecting the rest of your quality of living and saving for your future.
Sometimes it's alright to have beans on toast, even if they have to be organic.
The other really important thing is to not put all your shopping in one basket. Eg:
I only buy my body creams (usually £6) when they are on discount, I get through one a week (v dry skin), I am on six different websites until I see the discount then I order half a year's worth at once
Same with the deodorant my OH needs
Same with shower gel I need
I can tell you which of the main supermarkets does the best quality bin liners at the cheapest price at any one point in time
Same applies to cleaning products
Most of the time, own brand dishwasher tablets, laundry liquid etc are absolutely fine (the exception being fairy liquid) - do you buy big brands for all this stuff?
I don't buy shitty supermarket tin foil anymore, I am very aware of their price rises, I buy massive catering rolls of good quality that will last a year
I can see it's easily possible to spend what you have spent and when the OH and I were in London, we were doing over 1200 a month before I cracked down and now, if I really want to, I can get it below £600.
The devil is in the detail and the planning. Because I never allow myself to run out of anything I always have time to plan to buy lots of whatever that expensive thing is at the lowest possible price. Appreciate that might not be so practical in London with little storage space, but even when I lived there, I would still physically visit 4 to 5 different supermarkets a month to take advantage of discounts of things we need.
It takes time and planning but the effort is well worth it.