As far as snacking goes... you could do string cheese or turkey jerky too.

Regarding the expense of this diet... Stock up with the sales.

When tuna goes on sale 3 for $1.00 (or even 4) I hit the store. I buy meats in bulk and separate them out into meal size portions and freeze. Mayo, cheese, cream cheese, dip and dressings often go on sale buy on get one free or 2 for some set price. Eggs are a cheap alternative too.

Another $$$ saver idea.... make big batches of food and freeze. I don't know about England but here in the US you can get 100 peice plastic bowl set with lids for $10. You can make casseroles, beanless chili, veggie soup and crustless quiches in large quantities and freeze. They are perfect if you have a microwave at work or just need a quick easy lunch.