Some directories also have search tools.

http://www.yahoo.co.uk
Directory and search engine that allows UK only searches.

http://www.yell.co.uk
Full UK Yellow Pages.

http://www.ukdirectory.com
Small directory of UK web sites.

http://ukplus.co.uk
Extensive UK directory.

http://ifl2.rmplc.co.uk/pathways/pwpup.htm
RM's directory of sites, selected for their relevance to school students.