The above are both electronics suppliers. They have thousands of electronics parts, so you need to search about a bit for exactly the pieces you need. But these retailers often have better quality components at a much better price than guitar parts suppliers.
I've bought pickups from WD Music, and they seem to be a good company. They are completely guitar orientated too, so you can be sure you'll be getting the right part for the job. Maplin are a bit more general purpose, so although they seem cheaper, you'll have to check carefully to make sure the parts you get are up to spec. The stuff is usually good quality though, from my limited experience.
I've not had any experience of Guitar Spares or Rapidonline, but my first suggestion would have been WD Music. There may be cheaper suppliers out there, but (bearing in mind I'm not sure what an RS kit is, but assuming it's high quality) they are a reliable, sensibly priced bet.
'My Sound? Hell, if I can hear myself, that's my sound!' -Carl Perkins