Hi, I'm also based in the UK, and have a 5 gallon.
I bought an 18W 50/50 PC off of ebay, (although, maybe a 36W would be better, as you have twice the volume) for I think, ~£35. I have one sample of LPS (a torch coral) that has since split from 1 to 2 heads. I also have zoas, that have grown since purchase.
50/50 PC bulbI did, however, find the colours of the zoas a little dull so I added a strip of water proof blue LED's, again off ebay (for ~£7), that was already wired up to a plug, and the colours of everything improved. (Although it'd be well worth double checking with the seller as to whether the strip comes with the plug, as a friend of mine ordered some and had to buy a transformer separately). These strips seem to cause different reactions from different people, and I never have measured the light output with a PAR meter, that said, I love it and beleive it has only improved my tank.
ebay blue LED strip If you don't fancy putting together a custom LED build, based on my experience, then something similar to my set up would be more than suitable.