It's fine to bring food. Every British person I know who lives here comes back from their holidays with a car filled with half of Tesco 
particularly shelf-stable - I think there are actually rules about perishables (though they never check this IME).
English tea goes down well and they are very apprehensive when you explain it should be served with milk (make a special trip to a supermarket to ensure "Frischemilch" or you might be offered UHT or worse, coffee creamer).
Our biscuits are different to German ones - custard creams, bourbons, garibaldi, Nice, ginger snaps, jammy dodgers etc. Digestives and chocolate digestives are occasionally sold in German supermarkets so aren't as novel.
Mr. Kipling style mini pies, any of the Mr. Kipling type products or cake bars.
Pork pies. Scotch eggs. English bacon and sausages are quite different to German. (Though I think technically can't be imported) - you could make them a full English breakfast! Actually, I used to teach English and I did an exercise about foods people have/haven't tried, and I was really surprised that bacon was a common one that people had not tried - you can buy thin, American style streaky bacon here, and small squares of cooking bacon but that's it.
They can get baked beans here but they aren't as popular and they are expensive.
DS1 always likes all the "vintage" type sweets - refreshers, parma violets, love hearts, drumstick lollies, curly wurly. Germany has a million varieties of Haribo, so gummy sweets are common. You can buy Cadbury's here but generally only in the milk chocolate, caramel and maybe fruit and nut. All other Cadbury's bars are not sold, so they will be unusual.
English style large easter eggs are novel and they won't have seen these. Also the English Easter seasonal stuff is different to German - Creme eggs, Mini eggs. Though they might find creme eggs too sweet. Mini eggs are generally very popular.
Canned drinks if they have teenagers - Prime will be sought after. Tango is not sold here, and any Fanta other than Orange will be unknown. Maybe ginger beer, they don't really have that.
Crisps - Monster Munch, Wotsits, Skips, Snack a Jacks, Transform-A-Snack, Twiglets - these are all different.
Yes marmite is unique and marmalade is also different - will also spark a conversation about the German word "Marmalade" which is what we call jam/preserve.
If you have anything from your specific area - Pontefract cakes, Tablet, Kendal mint cake, Eccles cake etc - they will be intrigued by that.