Here are some links for you to look at.

LED Bulbs - LED Light Bulbs - Hydroponics | HIDHut Discount Hydroponics Supplies and Equipment
These are the LED bulbs I use for additional lighting and they work good. Its just that they are expensive.

I was also looking at these
25w Beagle GrowL�® LED Grow Light

Dont know how useful those links will be since you are in the UK. Gives you somewhere to start though.
GetThisOrDie Reviewed by GetThisOrDie on . Additional lighting Hi Guys, I am new on here and to growing so thanks for having me on board. I am in week six of my first grow using a 600w multi spectrum lamp. Which has done the job pretty well so far. My plants are now getting to the size where a few of the lower leaves are going yellow due to not getting enough light. Is it worth putting some extra lights in to light all the plant or as its my first grow not over complicating things and just cut the yellow leaves away. What additional lights do you guys Rating: 5