I've just had a quick workout based on the things i buy and came up with the following;
Weekly;
1kg Oats
8 pints Milk
7 Chicken Breasts (200g each)
1kg Frozen Sweetcorn
1kg Frozen Broccoli
1Kg Frozen Green Beans
30 large eggs
Loaf of bread
Spread/Butter
Instant Noodles x 5
Peanut Butter
Chocolate Powder
Sugar Free Jelly
Mandarin Segments x 2
This all comes to a little over £19 but doesn't include oil, salt & pepper, soy sauce, sesame oil, hp sauce and sundries like that. That's plenty for 75g oats twice a day, 4 eggs, 100g chicken with rice and veg, just realised i forgot to include the rice as i buy that in bulk but then i did include noodles as i tend to have chicken with veg and noodles instead of rice half the time. Plenty of milk, 50g peanut butter a day, no real treats in there but i did include the sugar free jelly with mandarin mixed in before it sets.
I don't stick to this as i have the odd meal my brother cooks or round my parents, maybe even a takeaway. I also tend to have other treats like i made a bunch of flapjacks and a batch of brownies.
I think that although £20 is tight if you plan ahead and get yourself organised you can do pretty well on £20 a week. Maybe rounding it up to £100 a month would make it a lot easier to include all the sundries.
It's certainly a good benchmark to aim for - then if you go over it or you need a treat then who cares.
I can give a breakdown of the above if needs be