Megalithic Portal has a huge database of prehistoric site information. It covers not just the UK, but the whole world. If you can't find what you are looking for on Prehistoric UK, try Megalithic Portal, you'll almost certainly find it there.
The Modern Antiquarian is a wonderful resource for those interested in Prehistoric sites. The site is based on Julian Copes epic book The Modern Antiquarian - a must for anyone interested in prehistoric sites of the Britain. There is loads of really good contributed material here as well, photos, folklore, all sorts.
This is site contains everything that you need to know about the Orkney islands. The Orkney islands contain some of the very best prehistoric sites in the UK. Well worth a visit - despite their remote location.
This is a really wonderful small site. The look of the site is nice and the navigation is easy - which is not the case with all sites! The descriptions of the sites are given from a very personal perspective - really nice.
This is a site full of content which the author(s) have actually visited themselves. The site covers both the UK and parts of Europe. The visuals on this site can be challenging, but don't let that put you off. There is loads of good stuff here.