What is the difference between Gammon and Bacon? Would it be generally reasonable to substitute the two as required?
I did my internship for culinary school in London, and if I recall correctly from what I saw in the markets there, what you call Gammon would be equivalent to our Ham - both coming from the pig's rear leg.
What we call bacon is what you'd call "streaky" or "streaky bacon" which is made from the pork belly.
Streaky would be the best substitute in recipes calling for bacon.
"Both ham and gammon are cut from the leg of a pig. The meat is the same but the preparation and treatment is different. Ham and gammon are both cured meats. This means that they are treated with salt, known as brining, and other substances before being eaten.
Ham is meat that is cut from the carcass and then treated.
Gammon is meat that is cut from the carcass after the brining treatment.
Both gammons and hams might also be smoked.
Traditionally, regional variants in the process and the ingredients used would bring different flavours to the cured meats. This explains the origin of distinctive varieties such as York ham, Bayonne ham and Prague ham.
Air dried hams are also cured first but then usually eaten raw whereas brined hams are baked or boiled before serving."
According to my local friendly butcher, Gammon is a type of bacon that is specifically from the hind quarters.
Bacon is defined as any pork that has been cured through a process of salting, either as a dry-cure or a wet-cure where the meat is either packed in salts or brine respectively. With wet curing, other ingredients can be added to impart other flavours, such as beer or sugars.
Typically, rashers are made from the body of the pig with streaky bacon coming from the belly, and back bacon coming from, well, the back (so the same cut as a pork loin chop). Bacon joints are typically made by combining cuts of bacon from the shoulder and collar, whilst the hind quarters are sold as gammon with a premium on price.
Traditionally ham referred to cooked gammon, although in modern uses, it is often extended to include other cooked bacon joints, which include moulded meats made from combining cuts together with other additives to help bind them.
Well bacon is a very generic term. You might want to clarify which you're referring to.
In America 'bacon' comes from the belly of the pig. Whereas, according to Wikipedia bacon in the UK typically comes from the back of the pig (we call this canadian bacon). Either way these are usually dry-cured.
Gammon cuts come from the hind legs of the pig. It is typically wet-cured.
Update: Based on your images above, yes gammon can be used as a substitute for back bacon.
In England good bacon is dry salted, (without sugar) and is mostly cut from the back and side of the pig and has a 'pork chop' appearance, with an eye (tenderloin), and muscle streaks and fat attached to the tail end (side of the ribs).
It is called a rasher, often has the skin attached, and can be either 'green' (unsmoked) or smoked (never hot smoked like in N. America).
It may be cooked to a crisp, or just till it turns opaque. Depends on taste.
There are many regional varieties, with Wiltshire and Danish being premium varieties. In Scotland, Ayrshire bacon is excellent and the rasher is rolled so the bacon rasher has a round appearance. Good bacon and gammon does not ooze white stuff, nor does it shrink much when it is fried.
Gammon is always cut thicker and is composed mostly of meat like a ham slice. It also tastes different. From my understanding it is usually made from salted leg meat.
You can find photos of 'English bacon rasher' and 'gammon rasher' on the web if you include UK in the search. If you haven't tasted this kind of bacon you can get an idea of the British bacon taste by salting a piece of pork belly in the fridge (it really needs fat to be delicious). Just leave out the sugar and spice. There are recipes on the web. You may never go back to the water soaked meat in a plastic bag that is sold as bacon in supermarkets.