(dg, you rang m'lady?)
Here I am, Fred, with my extensive knowledge of traditional British cuisine
Contrary to popular belief abroad, the main elements of British popular cuisine are Indian (or Bangladeshi or Pakistani). The foods you describe are Indian in nature.
A samosa is a tiny triangle of crispy pastry (filou?) wrapped around a spicy filling.Vegetable samosas have a mixture of vegetables inside, meat samosas have meat, which is usually lamb, inside. They are served as an appetiser.The triangles of pastry are cooked by being dropped in extremely hot oil for a few seconds, which makes them crisp and flaky.
A balti is a kind of clay oven, shaped like a big flask or bottle.The food is cooked by being pushed against the sides of it.It is fairly mildly spiced.The meat is marinated in spices before cooking. Over the last thirty years, this style of Indian cooking has become very popular here.It contrasts with the usual curry dishes, served with rice. It's normally served with naan bread.
What's this to do with Shell or Nobby's crisps ? Shell here is a reference to the oil company. It owns thousands of filling [gas] stations in Britain. Most of them sell food of one kind or another, sometimes to eat on site but mostly to take away.The reference to a Shell samosa will be to a samosa bought at the station which
may be reheated in a self-serve microwave or just taken away cold, as a snack.(The hint is that it's not a samosa of the highest quality but just a roadside snack, which may be a slur on Shell.It is unlikely to be quite as good as one eaten at table in a restaurant. On the other hand,good samosas are bought from street vendors in India, so why not?

)
The crisps (US 'potato chips') in Britain come in every flavour imaginable (The old joke was that one day the makers would make 'potato flavour'. They do now, but it's always in some fancy style e.g. cooked in some 'special' oil).One flavour made by the brand Nobby's will be flavoured to taste like chicken cooked in a balti oven, hence 'balti crisps'.
PS dg:You've surely found a very old promotion for Nobby's ! Running a pub is a thankless, and usually profitless, task for a newcomer now.