Have you seen the UK foods list...? There are loads of varieties there to keep you busy! But yes, I agree that it would be good to be able to try a couple of tins/pouches to see if they suit, before buying half a warehouse full and seeing it go to waste...
I'm not sure what our equivalent to Fancy Feast would be but I'm guessing it (FF) is a readily available supermarket brand so it may be equivalent to something like Whiskas, Felix or Sheba. I'm sure you've tried these. Some of our bigger supermarkets sell single pouches of other foods - if you live near a big Sainsbury's or Tesco, take a look (an hour in the pet food aisle is great use of your time!). Sainsbury's also sells a brand called Delicious - chicken and fish flavours - in both single pouches and boxes of six. I often recommend this to UKers as it's a good-quality low-carb food and easily available (ie as opposed to having to buy online). I think Tesco has something similar, the name escapes me but should be easy to find.
A lot of food choices depend on budget. The above-mentioned brands and supermarket own brands are cheap, but if you can spend more, have you tried Nature's Menu or Cheshire Cat, for example? Both are high-quality foods but probably twice as much as Whiskas etc.
The other thing you could try, if you haven't already, is getting some little tins of Encore or Applaws (both from
www.fetch.co.uk or some supermarkets) and just add a little of this on top of other foods you'd like your cats to eat. These tins are expensive too, but easier if you don't want to faff about with shredding real chicken every few days for the same purpose... same principle as adding tuna or Parmesan, ie once a cat gets a sniff of those on their food, they might go on to gobble the rest up. And still on that theme - I've been buying little sachets of cat soup to add to my kitty's food to try to up her liquid intake (she won't drink water or water added to her food, but she loves these soups, so you could add some of that to other stuff too).
If none of this works you may just have to accept that your cats have got you wrapped round their little paws and that half your life will be spent trying to please them and their very refined palates...
, so more lettuce for me and hopefully more meat for Mum. It all takes time. 
