Definitely try eBay sellers. Last time I looked (which is some time ago) there were quite a few UK-based sellers of chemistry. Not all aimed specifically at photography/printing, but that doesn't matter. In fact, I'd recommend not specifically looking for sellers/retailers who focus on these applications, but just look for the chemicals you need.
For salted paper, the salt itself you get from the local supermarket, and the silver nitrate from whomever has the best offer at this point. You don't need extremely pure silver nitrate for this purpose; generic 'technical grade' will do fine. It appears that there's currently a seller of silver nitrate in Italy; it's possible that even with shipping and duties/taxes the price per gram ends up cheaper than if you source locally.
Kallitype is often a bit of a challenge due to the ferric ammonium oxalate, which is kind of difficult to get. But the prints are very similar to Van Dyke, which uses commonly and cheaply available ferric ammonium citrate - the same stuff you use for classic cyanotype.
