Unfortunately there are no UK-based informational sites for diamond jewellery. There are plenty of diamond retailers that have some information on their websites but no single, complete, and objective resource.
There are some very good US-based sites however:
1. www.pricescope.com (Gary Holloway’s consumer awareness site)
2. www.diamondreview.com (and it’s identical site www.diamond.info)
3. www.diamondtalk.com
4. www.diamondcuttersintl.com (Fred Cuellar’s site, author of "How to Buy A Diamond”)
These sites all have forums where you can post a question and have it answered by experts who frequent the forums.