There's actually a sticky thread in the General Electronics Chat forum here. It's called "Where To Buy Electronic Components And Parts!" and looks like there are a couple of links for UK-based distributors. Take a look there.
What you're looking for is a switching regulator chip. Without getting into the theory here, they're DC-to-DC converters that exploit the properties of an inductor to generate a higher or lower voltage than the input. They're very high efficiency and can handle large currents. Some are made specifically to drive LEDs while others are for generic use. As long as the input voltage, output voltage, and current rating matches your needs, it doesn't matter too much which one you get.
To pick one out, you can go directly to a manufacturer's website (like MrAl recommended), find a part that fits your needs, then go to a distributor's website and see if they have that part. Alternatively, you can just go to the distributor's website and use their search engine and filter down to what you're looking for. All of the links in that sticky thread are distributors. When you search for switching regulators, they'll probably be categorized under "DC - DC converters" or "Power Management ICs" or something similar.