Be prepared for your shopping to take much longer than normal as you will be studying the labels and going back and forth. Just because something says low fat on it doesn't make it right and you start to appreciate just how complex food labelling is.
Your staples should be simple things where you don't need to study any labels. If you need to study the label chances are it's off the menu for now - definitely off the menu for attack! If you need to study labels, as well as checking for fat content per 100gr, avoid anything with sugar (sucrose, glucose, corn syrup, fructose, there are other names) and starch
My Staples tend to be (just studying my favourites on MySupermarket.com):
Oatbran (people have found Mornflake brand in Sainsbury, Morrisons, Tesco have an own brand one, Holland&Barret, health food or wholefood shops in general)
Fat-free fromage frais (0.1%)
Fresh Chicken breast
White fish fresh/frozen
Tinned tuna, mackerel, salmon (in water / brine ONLY)
Extra-lean beef mince or steak (<5%fat I think)
Cottage cheese - you can't get fat-free but the lowest are Tesco and Asda value ranges at less that 1% fat.
Plain Fat free yoghurt 0.1%
To make life more interesting:
chicken pieces (for roasting then take skin off before eating)
seafood in general
If you really need to: Vanilla or Toffee flavoured WW yoghurt or Muller lights (these are allowed max 2 per day as they're huge as well)
Diet drinks at less than 1cal per 100ml (max 3 cans a day I think)
Store cupboard items:
low-sodium soy sauce
low-fat/low salt stock cubes
if you buy spice mixes (like the Schwartz ones) keep an eye on the ingredients, they tend to contain sugar, so use very sparingly if at all.
granular sweetener like splenda
vanilla flavouring (avoid oil-based ones)
I'm sure I've forgotten something, no doubt others will chip in in a moment!