People who work on the deli do not usually solely work there. You have to progress to being on the counters. Generally, you will have to work on shop floor, stacking shelves and cleaning, and pick up the odd bit of training here and there. So you'll be a General Assistant, but learn deli skills so you can do overtime there.
Bakery staff positions are quite rare in Tesco, as they are very specialised areas and people generally work solely on bakery. The people who do learn the bakery skills are usually full time workers, who will be with the company for the foreseeable future. They don't tend to take part timers, because its such a hassle to train and retrain when people leave.
Dairy section is nothing special. They'll give you a position as a General Assistant, assigned to a particular section of the store. I am a General Assistant on the Provisions department at my store, so I work with milk occasionally, but mainly the chilled goods like cheese and ready meals. When you're a General Assistant, you will be till trained and expected to go down and help out during busy times.
Being trained on the tills is good if you'll ever want overtime, as most departments and bakeries and delis have very little in this regard. Checkouts always need people to do overtime though.