I have often found RS to be overpriced - sure, quality is generally good, but you will be paying top dollar for the RS association.
For general wiring stuff that we do here, we buy 100m rolls which can work out quite expensive if making a one off loom, especially as you would need a roll for each colour. For some of the wiring, where you only need a few meters of a particular colour, or even the cables with a striped identifier, I would suggest to make a list of the lengths you want and search for a thinwall automotive cable supplier through the likes of ebay as they will sell it by the meter rather than the roll. Buy what you can in one hit to save on the postage. For very heavy cable and audio cable, yes, spend a bit for some good quality, but the general purpose stuff, I`ve not noticed much difference between suppliers.