As for the cut, middle is acceptable if you can find it; if a little fiddly, size-wise, for a sandwich. But the real choice is between streaky and back. For me, back is the king of sandwich bacons. For three main reasons:
1) It offers the perfect ratio of meat to fat.
2) If you're using a square, white bread of normal dimensions, three slightly overlapped rashers of back provide the ideal volume of filling. You want a little bulk in there, not a flat plateau of pig.
3) Ideally, you would fry your bacon. But there is so much crap, watery bacon around that you may well end up having to grill it. If you do, it is almost impossible to grill streaky effectively without turning it into an oversized Frazzle: dry, brittle, and with none of the mellow, meaty flavour that you want. It is too unforgiving.
In a perfect world, you would have rashers of (dry-cured, outdoor bred, rare breed) back bacon, edged with up to an inch of fat, which, in a very hot pan, you would fry very briefly until the edges begin to curl golden brown. Do not incinerate it. You want a balance of succulent protein, marked with tangy patches of char, a perimeter of sweet, silky fat, and, at its edge, the primitive kick that only crisp, carbonised fat can deliver. If you need to grill it, grill it. But back bacon is still preferable
Bloaters are a type of whole cold-smoked herring. Bloaters are "salted and lightly smoked without gutting, giving a characteristic slightly gamey flavor" and are particularly associated with Great Yarmouth, England. Popular in the 19th and early 20th centuries, the food is now described as rare. Bloaters are sometimes called as Yarmouth bloater, or, jokingly, as a Yarmouth capon, two-eyed steak, or Billingsgate pheasant (after the Billingsgate Fish Market in London).[