The scampi butty is a frying problem put in a buttered roll. Scampi are breaded langoustine tails, deep-fried so the crumb crisps hard and the small piece of shellfish inside turns just opaque and sweet. They go into a soft white roll with butter and a cold sauce, tartare or ketchup, and almost nothing else. The defining fact of the build is that the value sits in the crumb: a good scampi is crisp-shelled and sweet inside, a bad one is a greasy nugget of breadcrumb around a chewy scrap, and the bread is arranged around defending that coating. Soft roll so it does not compete, cold sauce so it insulates rather than steams, the shortest possible trip from the fryer so the shell is still crisp on the first bite. Get the scampi right and the butty is honest; get it wrong and no amount of sauce rescues it.
The craft is contrast and the seal against the fry oil. The scampi bring all of the texture, hot, crisp, and salty, so the bread is chosen to be yielding and the sauce to be cold and sharp, a deliberate counter rather than an echo. Butter to the edges waterproofs the crumb so the oil weeping off the coating does not soak the base flat before it reaches the mouth. A roll is the usual carrier rather than sliced bread because scampi are small and round and roll loose: a soft roll cups them and holds the cluster together, where flat slices let them scatter and slide out the sides. The tartare goes in as a measured layer, capers and gherkin doing the acid work that cuts a hot fried filling, applied thin enough that it does not turn the crumb to paste.
The variations come off the same chip-shop counter. The fish-finger sandwich runs crumbed white fish through the identical logic; the fish butty swaps battered fish for crumbed; the saveloy and the spam fritter put a fried sausage or a battered slice of luncheon meat in the buttered roll the same way. Mushy peas, when they appear, are structural as much as flavour, a soft bed that stops the scampi shifting in the bread. Each of those deserves its own article rather than being crowded in here.