The branches in Złote Tarasy and Arkadia both have food departments but they are very very small compared to the food departments in the UK M&S shops. It's mainly dry/tinned goods but the ZT branch now has frozen meals too.
Marks does quite a good selection of food, including sauces and soups, frozen ready meals, wine and chocolates. It doesn't do any fresh food, so still no bacon, sadly.
The better section of food is in the Zlote Terasy Marks.
Yes but the management are considerate enough to sell M&S Hot English mustard. With enough of that smothered on the sausages, they're actually quite nice.
To be honest you can get most stuff from tesco , frisco.pl and as jonni said Kuchnie Świata but if you are really desperate for something that is not available then try here expatdirect.co.uk