From both personal experience and work, maplin has be a go to supplier, especially for parts, not just PCs, we lay our own cabling throughout the site. That includes power, networking, audio and visual. I ran my own Ethernet through out my house, for the ends of cables and small components, maplin has been great especially when you just need a couple of RJ45 ends and can't be bothered waiting a day or two for shipping.
Time to look for another go-to supplier...