Generally, lower temp is such a temp, that will melt thinner tin for smd devices as so quickly, as higher temp will do with tin with 2mm diam. Tin must be liquid properly to get good contact, but not to be oveburn. Try it at first at some old pcb, to get praxe. This is not something anybody can define too strictly. UK solder sevices are unknown for me, so I can't give you better advice.
For soldering points at lamp sockets or concept of soldering eyes at some pertinax desk you use higher temp, to heat it all properly. It is not pcb, so you can little raise temp. But allways use rosin. It is cheap but very usefull.
