Pork Sirloin vs Chump
Chump and loin are close together but they are very different cuts of meat. Chump is a working cut, so has much more connective tissue than loin, so a slower cooking process is recommended for it. It's not a bad cut at all, in fact it's delicious when prepared properly, you just need to adjust your method when using it. 

One thing to keep in mind is that pork loin has a chump end and a rib end, some people confuse chump end chops with chump, and it's not the same. Chump end chops are still loin, they just come from the back end of the loin and are therefore bigger. 

A sirloin chop is an American term, I don't hear it used in the UK generally. US and UK use different primal cuts. A sirloin pork chop (or steak, same thing) in the US contains several muscle groups. It's got hip, backbone area, a bit of loin, etc. It's a cheaper cut because it's got working meat in it and is tougher. It's a great choice for braising.