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.