Welcome to the forum.
Do you have a planted set up, i.e. live plants?
Tropical plants can take anything between 10-12 hours a day as they are used to the long light exposure. If you dont have plants, then anything goes really but i wouldnt really recommend anything over 12 hours as you may get problems with algae growth (if you dont have plants) and your fish need the dark times.
I have mine on 12pm-11pm and have live plants.
Not sure about Essex area, but Maidenhead Aquatics have various outlets, along with Aquajardin.