Other Supermarkets are available, even the 99p Store has enough supplies for a weekend trip.
Rat packs are usually much heavier than what you can knock up yourself.
Pasta 'mug shots', super noodles, one meal cous cous sachets, powdered mash will provide bulk of a main meal. Add in peperami, tuna pouches, dried peas/sweetcorn/mushrooms.
Bin the noodle flavour sachet and add half a cuppa soup for flavour.
Quaker or ready brek both do a 'just add hot water' porridge in sachet form. Add dried cranberries/fruit/nut/jam/honey to make it go a bit further.
Cereals/muesli and powdered milk.
For lunch/snacks fruit+nut, dried banana chips, m+M's, protein bars and gels. Pitta breads and jam, peanut butter, honey, cheese.
Only take cheese individually wrapped such as babybel etc.
These are great for peanut butter, jam, honey etc. As are the smaller sistema dressing pots.
http://tinyurl.com/pfvsr47
The flipside of carrying all this dried food is that you need to carry more water than if you just had 'boil in the bag' type meals. But I'd rather carry the extra weight in H2o form, than as gravy or custard.
Best to check what you can bring into the Country you're travelling to for restrictions, but you should be fine with most of what I listed.
Also carry a load of ziplock bags and decant the ingredients for each meal into one bag, then keep the whole lot in a dry bag. Best do this at the other end, as a ziplock bag of milk powder might arouse suspicion at customs!
I got most of these ideas from Paul Kirtley's blog and they have worked for me in the past.
http://paulkirtley.co.uk/2014/how-to-pack-enough-food-for-a-week-in-a-plce-...